summaryrefslogtreecommitdiffstats
path: root/Bugzilla/DB.pm
blob: 044aa3dea29707caa2df538453bfdfac3b26d5e2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
# -*- Mode: perl; indent-tabs-mode: nil -*-
#
# The contents of this file are subject to the Mozilla Public
# License Version 1.1 (the "License"); you may not use this file
# except in compliance with the License. You may obtain a copy of
# the License at http://www.mozilla.org/MPL/
#
# Software distributed under the License is distributed on an "AS
# IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
# implied. See the License for the specific language governing
# rights and limitations under the License.
#
# The Original Code is the Bugzilla Bug Tracking System.
#
# The Initial Developer of the Original Code is Netscape Communications
# Corporation. Portions created by Netscape are
# Copyright (C) 1998 Netscape Communications Corporation. All
# Rights Reserved.
#
# Contributor(s): Terry Weissman <terry@mozilla.org>
#                 Dan Mosedale <dmose@mozilla.org>
#                 Jacob Steenhagen <jake@bugzilla.org>
#                 Bradley Baetz <bbaetz@student.usyd.edu.au>
#                 Christopher Aillon <christopher@aillon.com>
#                 Tomas Kopal <Tomas.Kopal@altap.cz>
#                 Max Kanat-Alexander <mkanat@bugzilla.org>
#                 Lance Larsh <lance.larsh@oracle.com>

package Bugzilla::DB;

use strict;

use DBI;

# Inherit the DB class from DBI::db.
use base qw(DBI::db);

use Bugzilla::Config qw(:db);
use Bugzilla::Constants;
use Bugzilla::Util;
use Bugzilla::Error;
use Bugzilla::DB::Schema;

#####################################################################
# Constants
#####################################################################

use constant BLOB_TYPE => DBI::SQL_BLOB;

#####################################################################
# Connection Methods
#####################################################################

sub connect_shadow {
    my $params = Bugzilla->params;
    die "Tried to connect to non-existent shadowdb" 
        unless $params->{'shadowdb'};

    return _connect($db_driver, $params->{"shadowdbhost"},
                    $params->{'shadowdb'}, $params->{"shadowdbport"},
                    $params->{"shadowdbsock"}, $db_user, $db_pass);
}

sub connect_main {
    my ($no_db_name) = @_;
    my $connect_to_db = $db_name;
    $connect_to_db = "" if $no_db_name;
    return _connect($db_driver, $db_host, $connect_to_db, $db_port,
                    $db_sock, $db_user, $db_pass);
}

sub _connect {
    my ($driver, $host, $dbname, $port, $sock, $user, $pass) = @_;

    my $pkg_module = DB_MODULE->{lc($driver)}->{db};

    # do the actual import
    eval ("require $pkg_module")
        || die ("'$driver' is not a valid choice for \$db_driver in "
                . " localconfig: " . $@);

    # instantiate the correct DB specific module
    my $dbh = $pkg_module->new($user, $pass, $host, $dbname, $port, $sock);

    return $dbh;
}

sub _handle_error {
    require Carp;

    # Cut down the error string to a reasonable size
    $_[0] = substr($_[0], 0, 2000) . ' ... ' . substr($_[0], -2000)
        if length($_[0]) > 4000;
    $_[0] = Carp::longmess($_[0]);
    return 0; # Now let DBI handle raising the error
}

# List of abstract methods we are checking the derived class implements
our @_abstract_methods = qw(REQUIRED_VERSION PROGRAM_NAME DBD_VERSION
                            new sql_regexp sql_not_regexp sql_limit sql_to_days
                            sql_date_format sql_interval
                            bz_lock_tables bz_unlock_tables);

# This overridden import method will check implementation of inherited classes
# for missing implementation of abstract methods
# See http://perlmonks.thepen.com/44265.html
sub import {
    my $pkg = shift;

    # do not check this module
    if ($pkg ne __PACKAGE__) {
        # make sure all abstract methods are implemented
        foreach my $meth (@_abstract_methods) {
            $pkg->can($meth)
                or croak("Class $pkg does not define method $meth");
        }
    }

    # Now we want to call our superclass implementation.
    # If our superclass is Exporter, which is using caller() to find
    # a namespace to populate, we need to adjust for this extra call.
    # All this can go when we stop using deprecated functions.
    my $is_exporter = $pkg->isa('Exporter');
    $Exporter::ExportLevel++ if $is_exporter;
    $pkg->SUPER::import(@_);
    $Exporter::ExportLevel-- if $is_exporter;
}

sub sql_istrcmp {
    my ($self, $left, $right, $op) = @_;
    $op ||= "=";

    return $self->sql_istring($left) . " $op " . $self->sql_istring($right);
}

sub sql_istring {
    my ($self, $string) = @_;

    return "LOWER($string)";
}

sub sql_position {
    my ($self, $fragment, $text) = @_;

    return "POSITION($fragment IN $text)";
}

sub sql_group_by {
    my ($self, $needed_columns, $optional_columns) = @_;

    my $expression = "GROUP BY $needed_columns";
    $expression .= ", " . $optional_columns if defined($optional_columns);
    
    return $expression;
}

sub sql_string_concat {
    my ($self, @params) = @_;
    
    return '(' . join(' || ', @params) . ')';
}

sub sql_fulltext_search {
    my ($self, $column, $text) = @_;

    # This is as close as we can get to doing full text search using
    # standard ANSI SQL, without real full text search support. DB specific
    # modules should override this, as this will be always much slower.

    # make the string lowercase to do case insensitive search
    my $lower_text = lc($text);

    # split the text we search for into separate words
    my @words = split(/\s+/, $lower_text);

    # surround the words with wildcards and SQL quotes so we can use them
    # in LIKE search clauses
    @words = map($self->quote("%$_%"), @words);

    # untaint words, since they are safe to use now that we've quoted them
    map(trick_taint($_), @words);

    # turn the words into a set of LIKE search clauses
    @words = map("LOWER($column) LIKE $_", @words);

    # search for occurrences of all specified words in the column
    return "CASE WHEN (" . join(" AND ", @words) . ") THEN 1 ELSE 0 END";
}

#####################################################################
# General Info Methods
#####################################################################

# XXX - Needs to be documented.
sub bz_server_version {
    my ($self) = @_;
    return $self->get_info(18); # SQL_DBMS_VER
}

sub bz_last_key {
    my ($self, $table, $column) = @_;

    return $self->last_insert_id($db_name, undef, $table, $column);
}

sub bz_get_field_defs {
    my ($self) = @_;

    my $extra = "";
    if (!Bugzilla->user->in_group(Bugzilla->params->{'timetrackinggroup'})) {
        $extra = "AND name NOT IN ('estimated_time', 'remaining_time', " .
                 "'work_time', 'percentage_complete', 'deadline')";
    }

    my @fields;
    my $sth = $self->prepare("SELECT name, description FROM fielddefs
                              WHERE obsolete = 0 $extra
                              ORDER BY sortkey");
    $sth->execute();
    while (my $field_ref = $sth->fetchrow_hashref()) {
        push(@fields, $field_ref);
    }
    return(@fields);
}

#####################################################################
# Database Setup
#####################################################################

sub bz_setup_database {
    my ($self) = @_;

    # If we haven't ever stored a serialized schema,
    # set up the bz_schema table and store it.
    $self->_bz_init_schema_storage();
    
    my @desired_tables = $self->_bz_schema->get_table_list();

    foreach my $table_name (@desired_tables) {
        $self->bz_add_table($table_name);
    }
}

# The default implementation just returns what you passed-in. This function
# really exists just to be overridden in Bugzilla::DB::Mysql.
sub bz_enum_initial_values {
    my ($self, $enum_defaults) = @_;
    return $enum_defaults;
}

#####################################################################
# Schema Modification Methods
#####################################################################

sub bz_add_column {
    my ($self, $table, $name, $new_def, $init_value) = @_;

    # You can't add a NOT NULL column to a table with
    # no DEFAULT statement, unless you have an init_value.
    # SERIAL types are an exception, though, because they can
    # auto-populate.
    if ( $new_def->{NOTNULL} && !exists $new_def->{DEFAULT} 
         && !defined $init_value && $new_def->{TYPE} !~ /SERIAL/)
    {
        die "Failed adding the column ${table}.${name}:\n  You cannot add"
            . " a NOT NULL column with no default to an existing table,\n"
            . "  unless you specify something for \$init_value." 
    }

    my $current_def = $self->bz_column_info($table, $name);

    if (!$current_def) {
        my @statements = $self->_bz_real_schema->get_add_column_ddl(
            $table, $name, $new_def, 
            defined $init_value ? $self->quote($init_value) : undef);
        print "Adding new column $name to table $table ...\n";
        foreach my $sql (@statements) {
            $self->do($sql);
        }
        $self->_bz_real_schema->set_column($table, $name, $new_def);
        $self->_bz_store_real_schema;
    }
}

sub bz_alter_column {
    my ($self, $table, $name, $new_def, $set_nulls_to) = @_;

    my $current_def = $self->bz_column_info($table, $name);

    if (!$self->_bz_schema->columns_equal($current_def, $new_def)) {
        # You can't change a column to be NOT NULL if you have no DEFAULT
        # and no value for $set_nulls_to, if there are any NULL values 
        # in that column.
        if ($new_def->{NOTNULL} && 
            !exists $new_def->{DEFAULT} && !defined $set_nulls_to)
        {
            # Check for NULLs
            my $any_nulls = $self->selectrow_array(
                "SELECT 1 FROM $table WHERE $name IS NULL");
            if ($any_nulls) {
                die "You cannot alter the ${table}.${name} column to be"
                    . " NOT NULL without\nspecifying a default or"
                    . " something for \$set_nulls_to, because"
                    . " there are\nNULL values currently in it.";
            }
        }
        $self->bz_alter_column_raw($table, $name, $new_def, $current_def,
                                   $set_nulls_to);
        $self->_bz_real_schema->set_column($table, $name, $new_def);
        $self->_bz_store_real_schema;
    }
}


# bz_alter_column_raw($table, $name, $new_def, $current_def)
#
# Description: A helper function for bz_alter_column.
#              Alters a column in the database
#              without updating any Schema object. Generally
#              should only be called by bz_alter_column.
#              Used when either: (1) You don't yet have a Schema
#              object but you need to alter a column, for some reason.
#              (2) You need to alter a column for some database-specific
#              reason.
# Params:      $table   - The name of the table the column is on.
#              $name    - The name of the column you're changing.
#              $new_def - The abstract definition that you are changing
#                         this column to.
#              $current_def - (optional) The current definition of the
#                             column. Will be used in the output message,
#                             if given.
#              $set_nulls_to - The same as the param of the same name
#                              from bz_alter_column.
# Returns:     nothing
#
sub bz_alter_column_raw {
    my ($self, $table, $name, $new_def, $current_def, $set_nulls_to) = @_;
    my @statements = $self->_bz_real_schema->get_alter_column_ddl(
        $table, $name, $new_def,
        defined $set_nulls_to ? $self->quote($set_nulls_to) : undef);
    my $new_ddl = $self->_bz_schema->get_type_ddl($new_def);
    print "Updating column $name in table $table ...\n";
    if (defined $current_def) {
        my $old_ddl = $self->_bz_schema->get_type_ddl($current_def);
        print "Old: $old_ddl\n";
    }
    print "New: $new_ddl\n";
    $self->do($_) foreach (@statements);
}

sub bz_add_index {
    my ($self, $table, $name, $definition) = @_;

    my $index_exists = $self->bz_index_info($table, $name);

    if (!$index_exists) {
        $self->bz_add_index_raw($table, $name, $definition);
        $self->_bz_real_schema->set_index($table, $name, $definition);
        $self->_bz_store_real_schema;
    }
}

# bz_add_index_raw($table, $name, $silent)
#
# Description: A helper function for bz_add_index.
#              Adds an index to the database
#              without updating any Schema object. Generally
#              should only be called by bz_add_index.
#              Used when you don't yet have a Schema
#              object but you need to add an index, for some reason.
# Params:      $table  - The name of the table the index is on.
#              $name   - The name of the index you're adding.
#              $definition - The abstract index definition, in hashref
#                            or arrayref format.
#              $silent - (optional) If specified and true, don't output
#                        any message about this change.
# Returns:     nothing
#
sub bz_add_index_raw {
    my ($self, $table, $name, $definition, $silent) = @_;
    my @statements = $self->_bz_schema->get_add_index_ddl(
        $table, $name, $definition);
    print "Adding new index '$name' to the $table table ...\n" unless $silent;
    $self->do($_) foreach (@statements);
}

sub bz_add_table {
    my ($self, $name) = @_;

    my $table_exists = $self->bz_table_info($name);

    if (!$table_exists) {
        $self->_bz_add_table_raw($name);
        $self->_bz_real_schema->add_table($name,
            $self->_bz_schema->get_table_abstract($name));
        $self->_bz_store_real_schema;
    }
}

# _bz_add_table_raw($name) - Private
#
# Description: A helper function for bz_add_table.
#              Creates a table in the database without
#              updating any Schema object. Generally
#              should only be called by bz_add_table and by
#              _bz_init_schema_storage. Used when you don't
#              yet have a Schema object but you need to
#              add a table, for some reason.
# Params:      $name - The name of the table you're creating. 
#                  The definition for the table is pulled from 
#                  _bz_schema.
# Returns:     nothing
#
sub _bz_add_table_raw {
    my ($self, $name) = @_;
    my @statements = $self->_bz_schema->get_table_ddl($name);
    print "Adding new table $name ...\n";
    $self->do($_) foreach (@statements);
}

sub bz_drop_column {
    my ($self, $table, $column) = @_;

    my $current_def = $self->bz_column_info($table, $column);

    if ($current_def) {
        my @statements = $self->_bz_real_schema->get_drop_column_ddl(
            $table, $column);
        print "Deleting unused column $column from table $table ...\n";
        foreach my $sql (@statements) {
            # Because this is a deletion, we don't want to die hard if
            # we fail because of some local customization. If something
            # is already gone, that's fine with us!
            eval { $self->do($sql); } or warn "Failed SQL: [$sql] Error: $@";
        }
        $self->_bz_real_schema->delete_column($table, $column);
        $self->_bz_store_real_schema;
    }
}

sub bz_drop_index {
    my ($self, $table, $name) = @_;

    my $index_exists = $self->bz_index_info($table, $name);

    if ($index_exists) {
        $self->bz_drop_index_raw($table, $name);
        $self->_bz_real_schema->delete_index($table, $name);
        $self->_bz_store_real_schema;        
    }
}

# bz_drop_index_raw($table, $name, $silent)
#
# Description: A helper function for bz_drop_index.
#              Drops an index from the database
#              without updating any Schema object. Generally
#              should only be called by bz_drop_index.
#              Used when either: (1) You don't yet have a Schema 
#              object but you need to drop an index, for some reason.
#              (2) You need to drop an index that somehow got into the
#              database but doesn't exist in Schema.
# Params:      $table  - The name of the table the index is on.
#              $name   - The name of the index you're dropping.
#              $silent - (optional) If specified and true, don't output
#                        any message about this change.
# Returns:     nothing
#
sub bz_drop_index_raw {
    my ($self, $table, $name, $silent) = @_;
    my @statements = $self->_bz_schema->get_drop_index_ddl(
        $table, $name);
    print "Removing index '$name' from the $table table...\n" unless $silent;
    foreach my $sql (@statements) {
        # Because this is a deletion, we don't want to die hard if
        # we fail because of some local customization. If something
        # is already gone, that's fine with us!
        eval { $self->do($sql) } or warn "Failed SQL: [$sql] Error: $@";
    }
}

sub bz_drop_table {
    my ($self, $name) = @_;

    my $table_exists = $self->bz_table_info($name);

    if ($table_exists) {
        my @statements = $self->_bz_schema->get_drop_table_ddl($name);
        print "Dropping table $name...\n";
        foreach my $sql (@statements) {
            # Because this is a deletion, we don't want to die hard if
            # we fail because of some local customization. If something
            # is already gone, that's fine with us!
            eval { $self->do($sql); } or warn "Failed SQL: [$sql] Error: $@";
        }
        $self->_bz_real_schema->delete_table($name);
        $self->_bz_store_real_schema;
    }
}

sub bz_rename_column {
    my ($self, $table, $old_name, $new_name) = @_;

    my $old_col_exists  = $self->bz_column_info($table, $old_name);

    if ($old_col_exists) {
        my $already_renamed = $self->bz_column_info($table, $new_name);
        die "Name conflict: Cannot rename ${table}.${old_name} to"
            . " ${table}.${new_name},\nbecause ${table}.${new_name}"
            . " already exists." if $already_renamed;
        my @statements = $self->_bz_real_schema->get_rename_column_ddl(
            $table, $old_name, $new_name);
        print "Changing column $old_name in table $table to"
              . " be named $new_name...\n";
        foreach my $sql (@statements) {
            $self->do($sql);
        }
        $self->_bz_real_schema->rename_column($table, $old_name, $new_name);
        $self->_bz_store_real_schema;
    }
}

#####################################################################
# Schema Information Methods
#####################################################################

sub _bz_schema {
    my ($self) = @_;
    return $self->{private_bz_schema} if exists $self->{private_bz_schema};
    my @module_parts = split('::', ref $self);
    my $module_name  = pop @module_parts;
    $self->{private_bz_schema} = Bugzilla::DB::Schema->new($module_name);
    return $self->{private_bz_schema};
}

# _bz_get_initial_schema()
#
# Description: A protected method, intended for use only by Bugzilla::DB
#              and subclasses. Used to get the initial Schema that will
#              be written to disk for _bz_init_schema_storage. You probably
#              want to use _bz_schema or _bz_real_schema instead of this
#              method.
# Params:      none
# Returns:     A Schema object that can be serialized and written to disk
#              for _bz_init_schema_storage.
sub _bz_get_initial_schema {
    my ($self) = @_;
    return $self->_bz_schema->get_empty_schema();
}

sub bz_column_info {
    my ($self, $table, $column) = @_;
    return $self->_bz_real_schema->get_column_abstract($table, $column);
}

sub bz_index_info {
    my ($self, $table, $index) = @_;
    my $index_def =
        $self->_bz_real_schema->get_index_abstract($table, $index);
    if (ref($index_def) eq 'ARRAY') {
        $index_def = {FIELDS => $index_def, TYPE => ''};
    }
    return $index_def;
}

sub bz_table_info {
    my ($self, $table) = @_;
    return $self->_bz_real_schema->get_table_abstract($table);
}


sub bz_table_columns {
    my ($self, $table) = @_;
    return $self->_bz_real_schema->get_table_columns($table);
}

#####################################################################
# Protected "Real Database" Schema Information Methods
#####################################################################

# Only Bugzilla::DB and subclasses should use these methods.
# If you need a method that does the same thing as one of these
# methods, use the version without _real on the end.

# bz_table_columns_real($table)
#
# Description: Returns a list of columns on a given table
#              as the table actually is, on the disk.
# Params:      $table - Name of the table.
# Returns:     An array of column names.
#
sub bz_table_columns_real {
    my ($self, $table) = @_;
    my $sth = $self->column_info(undef, undef, $table, '%');
    return @{ $self->selectcol_arrayref($sth, {Columns => [4]}) };
}

# bz_table_list_real()
#
# Description: Gets a list of tables in the current
#              database, directly from the disk.
# Params:      none
# Returns:     An array containing table names.
sub bz_table_list_real {
    my ($self) = @_;
    my $table_sth = $self->table_info(undef, undef, undef, "TABLE");
    return @{$self->selectcol_arrayref($table_sth, { Columns => [3] })};
}

#####################################################################
# Transaction Methods
#####################################################################

sub bz_start_transaction {
    my ($self) = @_;

    if ($self->{private_bz_in_transaction}) {
        ThrowCodeError("nested_transaction");
    } else {
        # Turn AutoCommit off and start a new transaction
        $self->begin_work();
        $self->{private_bz_in_transaction} = 1;
    }
}

sub bz_commit_transaction {
    my ($self) = @_;

    if (!$self->{private_bz_in_transaction}) {
        ThrowCodeError("not_in_transaction");
    } else {
        $self->commit();

        $self->{private_bz_in_transaction} = 0;
    }
}

sub bz_rollback_transaction {
    my ($self) = @_;

    if (!$self->{private_bz_in_transaction}) {
        ThrowCodeError("not_in_transaction");
    } else {
        $self->rollback();

        $self->{private_bz_in_transaction} = 0;
    }
}

#####################################################################
# Subclass Helpers
#####################################################################

sub db_new {
    my ($class, $dsn, $user, $pass, $attributes) = @_;

    # set up default attributes used to connect to the database
    # (if not defined by DB specific implementation)
    $attributes = { RaiseError => 0,
                    AutoCommit => 1,
                    PrintError => 0,
                    ShowErrorStatement => 1,
                    HandleError => \&_handle_error,
                    TaintIn => 1,
                    FetchHashKeyName => 'NAME',  
                    # Note: NAME_lc causes crash on ActiveState Perl
                    # 5.8.4 (see Bug 253696)
                    # XXX - This will likely cause problems in DB
                    # back ends that twiddle column case (Oracle?)
                  } if (!defined($attributes));

    # connect using our known info to the specified db
    # Apache::DBI will cache this when using mod_perl
    my $self = DBI->connect($dsn, $user, $pass, $attributes)
        or die "\nCan't connect to the database.\nError: $DBI::errstr\n"
        . "  Is your database installed and up and running?\n  Do you have"
        . " the correct username and password selected in localconfig?\n\n";

    # RaiseError was only set to 0 so that we could catch the 
    # above "die" condition.
    $self->{RaiseError} = 1;

    # class variables
    $self->{private_bz_in_transaction} = 0;

    bless ($self, $class);

    return $self;
}

#####################################################################
# Private Methods
#####################################################################

=begin private

=head1 PRIVATE METHODS

These methods really are private. Do not override them in subclasses.

=over 4

=item C<_init_bz_schema_storage>

 Description: Initializes the bz_schema table if it contains nothing.
 Params:      none
 Returns:     nothing

=cut

sub _bz_init_schema_storage {
    my ($self) = @_;

    my $table_size;
    eval {
        $table_size = 
            $self->selectrow_array("SELECT COUNT(*) FROM bz_schema");
    };

    if (!$table_size) {
        my $init_schema = $self->_bz_get_initial_schema;
        my $store_me = $init_schema->serialize_abstract();
        my $schema_version = $init_schema->SCHEMA_VERSION;

        # If table_size is not defined, then we hit an error reading the
        # bz_schema table, which means it probably doesn't exist yet. So,
        # we have to create it. If we failed above for some other reason,
        # we'll see the failure here.
        # However, we must create the table after we do get_initial_schema,
        # because some versions of get_initial_schema read that the table
        # exists and then add it to the Schema, where other versions don't.
        if (!defined $table_size) {
            $self->_bz_add_table_raw('bz_schema');
        }

        print "Initializing the new Schema storage...\n";
        my $sth = $self->prepare("INSERT INTO bz_schema "
                                 ." (schema_data, version) VALUES (?,?)");
        $sth->bind_param(1, $store_me, $self->BLOB_TYPE);
        $sth->bind_param(2, $schema_version);
        $sth->execute();

        # And now we have to update the on-disk schema to hold the bz_schema
        # table, if the bz_schema table didn't exist when we were called.
        if (!defined $table_size) {
            $self->_bz_real_schema->add_table('bz_schema',
                $self->_bz_schema->get_table_abstract('bz_schema'));
            $self->_bz_store_real_schema;
        }
    } 
    # Sanity check
    elsif ($table_size > 1) {
        # We tell them to delete the newer one. Better to have checksetup
        # run migration code too many times than to have it not run the
        # correct migration code at all.
        die "Attempted to initialize the schema but there are already "
            . " $table_size copies of it stored.\nThis should never happen.\n"
            . " Compare the rows of the bz_schema table and delete the "
            . "newer one(s).";
    }
}

=item C<_bz_real_schema()>

 Description: Returns a Schema object representing the database
              that is being used in the current installation.
 Params:      none
 Returns:     A C<Bugzilla::DB::Schema> object representing the database
              as it exists on the disk.

=cut

sub _bz_real_schema {
    my ($self) = @_;
    return $self->{private_real_schema} if exists $self->{private_real_schema};

    my ($data, $version) = $self->selectrow_array(
        "SELECT schema_data, version FROM bz_schema");

    (die "_bz_real_schema tried to read the bz_schema table but it's empty!")
        if !$data;

    $self->{private_real_schema} = 
        $self->_bz_schema->deserialize_abstract($data, $version);

    return $self->{private_real_schema};
}

=item C<_bz_store_real_schema()>

 Description: Stores the _bz_real_schema structures in the database
              for later recovery. Call this function whenever you make
              a change to the _bz_real_schema.
 Params:      none
 Returns:     nothing

 Precondition: $self->{_bz_real_schema} must exist.

=back

=end private

=cut

sub _bz_store_real_schema {
    my ($self) = @_;

    # Make sure that there's a schema to update
    my $table_size = $self->selectrow_array("SELECT COUNT(*) FROM bz_schema");

    die "Attempted to update the bz_schema table but there's nothing "
        . "there to update. Run checksetup." unless $table_size;

    # We want to store the current object, not one
    # that we read from the database. So we use the actual hash
    # member instead of the subroutine call. If the hash
    # member is not defined, we will (and should) fail.
    my $update_schema = $self->{private_real_schema};
    my $store_me = $update_schema->serialize_abstract();
    my $schema_version = $update_schema->SCHEMA_VERSION;
    my $sth = $self->prepare("UPDATE bz_schema 
                                 SET schema_data = ?, version = ?");
    $sth->bind_param(1, $store_me, $self->BLOB_TYPE);
    $sth->bind_param(2, $schema_version);
    $sth->execute();
}

1;

__END__

=head1 NAME

Bugzilla::DB - Database access routines, using L<DBI>

=head1 SYNOPSIS

  # Obtain db handle
  use Bugzilla::DB;
  my $dbh = Bugzilla->dbh;

  # prepare a query using DB methods
  my $sth = $dbh->prepare("SELECT " .
                          $dbh->sql_date_format("creation_ts", "%Y%m%d") .
                          " FROM bugs WHERE bug_status != 'RESOLVED' " .
                          $dbh->sql_limit(1));

  # Execute the query
  $sth->execute;

  # Get the results
  my @result = $sth->fetchrow_array;

  # Schema Modification
  $dbh->bz_add_column($table, $name, \%definition, $init_value);
  $dbh->bz_add_index($table, $name, $definition);
  $dbh->bz_add_table($name);
  $dbh->bz_drop_index($table, $name);
  $dbh->bz_drop_table($name);
  $dbh->bz_alter_column($table, $name, \%new_def, $set_nulls_to);
  $dbh->bz_drop_column($table, $column);
  $dbh->bz_rename_column($table, $old_name, $new_name);

  # Schema Information
  my $column = $dbh->bz_column_info($table, $column);
  my $index  = $dbh->bz_index_info($table, $index);

  # General Information
  my @fields    = $dbh->bz_get_field_defs();

=head1 DESCRIPTION

Functions in this module allows creation of a database handle to connect
to the Bugzilla database. This should never be done directly; all users
should use the L<Bugzilla> module to access the current C<dbh> instead.

This module also contains methods extending the returned handle with
functionality which is different between databases allowing for easy
customization for particular database via inheritance. These methods
should be always preffered over hard-coding SQL commands.

=head1 CONSTANTS

Subclasses of Bugzilla::DB are required to define certain constants. These
constants are required to be subroutines or "use constant" variables.

=over 4

=item C<BLOB_TYPE>

The C<\%attr> argument that must be passed to bind_param in order to 
correctly escape a C<LONGBLOB> type.

=back


=head1 CONNECTION

A new database handle to the required database can be created using this
module. This is normally done by the L<Bugzilla> module, and so these routines
should not be called from anywhere else.

=head2 Functions

=over 4

=item C<connect_main>

 Description: Function to connect to the main database, returning a new
              database handle.
 Params:      $no_db_name (optional) - If true, Connect to the database 
                  server, but don't connect to a specific database. This 
                  is only used when creating a database. After you create
                  the database, you should re-create a new Bugzilla::DB object
                  without using this parameter. 
 Returns:     new instance of the DB class

=item C<connect_shadow>

 Description: Function to connect to the shadow database, returning a new
              database handle.
              This routine C<die>s if no shadow database is configured.
 Params:      none
 Returns:     new instance of the DB class

=item C<_connect>

 Description: Internal function, creates and returns a new, connected
              instance of the correct DB class.
              This routine C<die>s if no driver is specified.
 Params:      $driver = name of the database driver to use
              $host = host running the database we are connecting to
              $dbname = name of the database to connect to
              $port = port the database is listening on
              $sock = socket the database is listening on
              $user = username used to log in to the database
              $pass = password used to log in to the database
 Returns:     new instance of the DB class

=item C<_handle_error>

 Description: Function passed to the DBI::connect call for error handling.
              It shortens the error for printing.

=item C<import>

 Description: Overrides the standard import method to check that derived class
              implements all required abstract methods. Also calls original
              implementation in its super class.

=back


=head1 ABSTRACT METHODS

Note: Methods which can be implemented generically for all DBs are implemented in
this module. If needed, they can be overridden with DB specific code.
Methods which do not have standard implementation are abstract and must
be implemented for all supported databases separately.
To avoid confusion with standard DBI methods, all methods returning string with
formatted SQL command have prefix C<sql_>. All other methods have prefix C<bz_>.

=over 4

=item C<new>

 Description: Constructor
              Abstract method, should be overridden by database specific code.
 Params:      $user = username used to log in to the database
              $pass = password used to log in to the database
              $host = host running the database we are connecting to
              $dbname = name of the database to connect to
              $port = port the database is listening on
              $sock = socket the database is listening on
 Returns:     new instance of the DB class
 Note:        The constructor should create a DSN from the parameters provided and
              then call C<db_new()> method of its super class to create a new
              class instance. See C<db_new> description in this module. As per
              DBI documentation, all class variables must be prefixed with
              "private_". See L<DBI>.

=item C<sql_regexp>

 Description: Outputs SQL regular expression operator for POSIX regex
              searches (case insensitive) in format suitable for a given
              database.
              Abstract method, should be overridden by database specific code.
 Params:      $expr = SQL expression for the text to be searched (scalar)
              $pattern = the regular expression to search for (scalar)
 Returns:     formatted SQL for regular expression search (e.g. REGEXP)
              (scalar)

=item C<sql_not_regexp>

 Description: Outputs SQL regular expression operator for negative POSIX
              regex searches (case insensitive) in format suitable for a given
              database.
              Abstract method, should be overridden by database specific code.
 Params:      $expr = SQL expression for the text to be searched (scalar)
              $pattern = the regular expression to search for (scalar)
 Returns:     formatted SQL for negative regular expression search
              (e.g. NOT REGEXP) (scalar)

=item C<sql_limit>

 Description: Returns SQL syntax for limiting results to some number of rows
              with optional offset if not starting from the begining.
              Abstract method, should be overridden by database specific code.
 Params:      $limit = number of rows to return from query (scalar)
              $offset = number of rows to skip prior counting (scalar)
 Returns:     formatted SQL for limiting number of rows returned from query
              with optional offset (e.g. LIMIT 1, 1) (scalar)

=item C<sql_from_days>

 Description: Outputs SQL syntax for converting Julian days to date.
              Abstract method, should be overridden by database specific code.
 Params:      $days = days to convert to date
 Returns:     formatted SQL for returning Julian days in dates. (scalar)

=item C<sql_to_days>

 Description: Outputs SQL syntax for converting date to Julian days.
              Abstract method, should be overridden by database specific code.
 Params:      $date = date to convert to days
 Returns:     formatted SQL for returning date fields in Julian days. (scalar)

=item C<sql_date_format>

 Description: Outputs SQL syntax for formatting dates.
              Abstract method, should be overridden by database specific code.
 Params:      $date = date or name of date type column (scalar)
              $format = format string for date output (scalar)
              (%Y = year, four digits, %y = year, two digits, %m = month,
               %d = day, %a = weekday name, 3 letters, %H = hour 00-23,
               %i = minute, %s = second)
 Returns:     formatted SQL for date formatting (scalar)

=item C<sql_interval>

 Description: Outputs proper SQL syntax for a time interval function.
              Abstract method, should be overridden by database specific code.
 Params:      $interval - the time interval requested (e.g. '30') (integer)
              $units    - the units the interval is in (e.g. 'MINUTE') (string)
 Returns:     formatted SQL for interval function (scalar)

=item C<sql_position>

 Description: Outputs proper SQL syntax determinig position of a substring
              (fragment) withing a string (text). Note: if the substring or
              text are string constants, they must be properly quoted
              (e.g. "'pattern'").
 Params:      $fragment = the string fragment we are searching for (scalar)
              $text = the text to search (scalar)
 Returns:     formatted SQL for substring search (scalar)

=item C<sql_group_by>

 Description: Outputs proper SQL syntax for grouping the result of a query.
              For ANSI SQL databases, we need to group by all columns we are
              querying for (except for columns used in aggregate functions).
              Some databases require (or even allow) to specify only one
              or few columns if the result is uniquely defined. For those
              databases, the default implementation needs to be overloaded.
 Params:      $needed_columns = string with comma separated list of columns
              we need to group by to get expected result (scalar)
              $optional_columns = string with comma separated list of all
              other columns we are querying for, but which are not in the
              required list.
 Returns:     formatted SQL for row grouping (scalar)

=item C<sql_string_concat>

 Description: Returns SQL syntax for concatenating multiple strings (constants
              or values from table columns) together.
 Params:      @params = array of column names or strings to concatenate
 Returns:     formatted SQL for concatenating specified strings

=item C<sql_fulltext_search>

 Description: Returns SQL syntax for performing a full text search for
              specified text on a given column.
              There is a ANSI SQL version of this method implemented using
              LIKE operator, but it's not a real full text search. DB specific
              modules should override this, as this generic implementation will
              be always much slower. This generic implementation returns
              'relevance' as 0 for no match, or 1 for a match.
 Params:      $column = name of column to search (scalar)
              $text = text to search for (scalar)
 Returns:     formatted SQL for full text search

=item C<sql_istrcmp>

 Description: Returns SQL for a case-insensitive string comparison.
 Params:      $left - What should be on the left-hand-side of the
                      operation.
              $right - What should be on the right-hand-side of the
                       operation.
              $op (optional) - What the operation is. Should be a 
                  valid ANSI SQL comparison operator, like "=", "<", 
                  "LIKE", etc. Defaults to "=" if not specified.
 Returns:     A SQL statement that will run the comparison in 
              a case-insensitive fashion.
 Note:        Uses sql_istring, so it has the same performance concerns.
              Try to avoid using this function unless absolutely necessary.
              Subclass Implementors: Override sql_istring instead of this
              function, most of the time (this function uses sql_istring).

=item C<sql_istring>

 Description: Returns SQL syntax "preparing" a string or text column for
              case-insensitive comparison. 
 Params:      $string - string to convert (scalar)
 Returns:     formatted SQL making the string case insensitive
 Note:        The default implementation simply calls LOWER on the parameter.
              If this is used to search on a text column with index, the index
              will not be usually used unless it was created as LOWER(column).

=item C<bz_lock_tables>

 Description: Performs a table lock operation on specified tables.
              If the underlying database supports transactions, it should also
              implicitly start a new transaction.
              Abstract method, should be overridden by database specific code.
 Params:      @tables = list of names of tables to lock in MySQL
              notation (ex. 'bugs AS bugs2 READ', 'logincookies WRITE')
 Returns:     none

=item C<bz_unlock_tables>

 Description: Performs a table unlock operation
              If the underlying database supports transactions, it should also
              implicitly commit or rollback the transaction.
              Also, this function should allow to be called with the abort flag
              set even without locking tables first without raising an error
              to simplify error handling.
              Abstract method, should be overridden by database specific code.
 Params:      $abort = UNLOCK_ABORT (true, 1) if the operation on locked tables
              failed (if transactions are supported, the action will be rolled
              back). False (0) or no param if the operation succeeded.
 Returns:     none

=back


=head1 IMPLEMENTED METHODS

These methods are implemented in Bugzilla::DB, and only need
to be implemented in subclasses if you need to override them for 
database-compatibility reasons.

=head2 General Information Methods

These methods return information about data in the database.

=over 4

=item C<bz_last_key>

 Description: Returns the last serial number, usually from a previous INSERT.
              Must be executed directly following the relevant INSERT.
              This base implementation uses DBI->last_insert_id. If the
              DBD supports it, it is the preffered way to obtain the last
              serial index. If it is not supported, the DB specific code
              needs to override it with DB specific code.
 Params:      $table = name of table containing serial column (scalar)
              $column = name of column containing serial data type (scalar)
 Returns:     Last inserted ID (scalar)

=back


=head2 Database Setup Methods

These methods are used by the Bugzilla installation programs to set up
the database.

=over 4

=item C<bz_enum_initial_values(\%enum_defaults)>

 Description: For an upgrade or an initial installation, provides
              what the values should be for the "enum"-type fields,
              such as version, op_sys, rep_platform, etc.
 Params:      \%enum_defaults - The default initial list of values for
                                each enum field. A hash, with the field
                                names pointing to an arrayref of values.
 Returns:     A hashref with the correct initial values for the enum fields.

=back


=head2 Schema Modification Methods

These methods modify the current Bugzilla Schema.

Where a parameter says "Abstract index/column definition", it returns/takes
information in the formats defined for indexes and columns in
C<Bugzilla::DB::Schema::ABSTRACT_SCHEMA>.

=over 4

=item C<bz_add_column($table, $name, \%definition, $init_value)>

 Description: Adds a new column to a table in the database. Prints out
              a brief statement that it did so, to stdout.
              Note that you cannot add a NOT NULL column that has no
              default -- the database won't know what to set all
              the NOT NULL values to.
 Params:      $table = the table where the column is being added
              $name  = the name of the new column
              \%definition = Abstract column definition for the new column
              $init_value = (optional) An initial value to set the column
                            to. Required if your column is NOT NULL and has
                            no DEFAULT set.
 Returns:     nothing

=item C<bz_add_index($table, $name, $definition)>

 Description: Adds a new index to a table in the database. Prints
              out a brief statement that it did so, to stdout.
              If the index already exists, we will do nothing.
 Params:      $table - The table the new index is on.
              $name  - A name for the new index.
              $definition - An abstract index definition. 
                            Either a hashref or an arrayref.
 Returns:     nothing

=item C<bz_add_table($name)>

 Description: Creates a new table in the database, based on the
              definition for that table in the abstract schema.
              Note that unlike the other 'add' functions, this does
              not take a definition, but always creates the table
              as it exists in the ABSTRACT_SCHEMA.
              If a table with that name already exists, then this
              function returns silently.
 Params:      $name - The name of the table you want to create.
 Returns:     nothing

=item C<bz_drop_index($table, $name)>

 Description: Removes an index from the database. Prints out a brief
              statement that it did so, to stdout. If the index
              doesn't exist, we do nothing.
 Params:      $table - The table that the index is on.
              $name  - The name of the index that you want to drop.
 Returns:     nothing

=item C<bz_drop_table($name)>

 Description: Drops a table from the database. If the table
              doesn't exist, we just return silently.
 Params:      $name - The name of the table to drop.
 Returns:     nothing

=item C<bz_alter_column($table, $name, \%new_def, $set_nulls_to)>

 Description: Changes the data type of a column in a table. Prints out
              the changes being made to stdout. If the new type is the
              same as the old type, the function returns without changing
              anything.
 Params:      $table   = the table where the column is
              $name    = the name of the column you want to change
              $new_def = An abstract column definition for the new 
                         data type of the columm
              $set_nulls_to = (Optional) If you are changing the column
                              to be NOT NULL, you probably also want to
                              set any existing NULL columns to a particular
                              value. Specify that value here. 
                              NOTE: The value should not already be SQL-quoted.
 Returns:     nothing

=item C<bz_drop_column($table, $column)>

 Description: Removes a column from a database table. If the column
              doesn't exist, we return without doing anything. If we do
              anything, we print a short message to stdout about the change.
 Params:      $table  = The table where the column is
              $column = The name of the column you want to drop
 Returns:     none

=item C<bz_rename_column($table, $old_name, $new_name)>

 Description: Renames a column in a database table. If the C<$old_name>
              column doesn't exist, we return without doing anything.
              If C<$old_name> and C<$new_name> both already exist in the
              table specified, we fail.
 Params:      $table    = The table containing the column 
                          that you want to rename
              $old_name = The current name of the column that 
                          you want to rename
              $new_name = The new name of the column
 Returns:     nothing

=back


=head2 Schema Information Methods

These methods return information about the current Bugzilla database
schema, as it currently exists on the disk. 

Where a parameter says "Abstract index/column definition", it returns/takes
information in the formats defined for indexes and columns in
C<Bugzilla::DB::Schema::ABSTRACT_SCHEMA>.

=over 4

=item C<bz_column_info($table, $column)>

 Description: Get abstract column definition.
 Params:      $table - The name of the table the column is in.
              $column - The name of the column.
 Returns:     An abstract column definition for that column.
              If the table or column does not exist, we return undef.

=item C<bz_index_info($table, $index)>

 Description: Get abstract index definition.
 Params:      $table - The table the index is on.
              $index - The name of the index.
 Returns:     An abstract index definition for that index,
              always in hashref format. The hashref will
              always contain the TYPE element, but it will
              be an empty string if it's just a normal index.
              If the index does not exist, we return undef.

=back


=head2 Deprecated Schema Information Methods

These methods return info about the current Bugzilla database, for
MySQL only.

=over 4

=item C<bz_get_field_defs>

 Description: Returns a list of all the "bug" fields in Bugzilla. The list
              contains hashes, with a 'name' key and a 'description' key.
 Params:      none
 Returns:     List of all the "bug" fields

=back


=head2 Transaction Methods

These methods deal with the starting and stopping of transactions 
in the database.

=over 4

=item C<bz_start_transaction>

 Description: Starts a transaction if supported by the database being used
 Params:      none
 Returns:     none

=item C<bz_commit_transaction>

 Description: Ends a transaction, commiting all changes, if supported by
              the database being used
 Params:      none
 Returns:     none

=item C<bz_rollback_transaction>

 Description: Ends a transaction, rolling back all changes, if supported by
              the database being used
 Params:      none
 Returns:     none

=back


=head1 SUBCLASS HELPERS

Methods in this class are intended to be used by subclasses to help them
with their functions.

=over 4

=item C<db_new>

 Description: Constructor
 Params:      $dsn = database connection string
              $user = username used to log in to the database
              $pass = password used to log in to the database
              $attributes = set of attributes for DB connection (optional)
 Returns:     new instance of the DB class
 Note:        the name of this constructor is not new, as that would make
              our check for implementation of new() by derived class useles.

=back


=head1 SEE ALSO

L<DBI>

L<Bugzilla::Constants> - The C<DB_MODULE> constant.

=cut