Merge 'trunk' into 'pri_key_refactor'

[dbsrgits/DBIx-Class.git] / lib / DBIx / Class / Storage / DBI / MSSQL.pm

diff --git a/lib/DBIx/Class/Storage/DBI/MSSQL.pm b/lib/DBIx/Class/Storage/DBI/MSSQL.pm
index 178a007..fedec7f 100644 (file)
--- a/lib/DBIx/Class/Storage/DBI/MSSQL.pm
+++ b/lib/DBIx/Class/Storage/DBI/MSSQL.pm
@@ -3,7 +3,7 @@ package DBIx::Class::Storage::DBI::MSSQL;
 use strict;
 use warnings;
 
-use base qw/DBIx::Class::Storage::DBI::AmbiguousGlob DBIx::Class::Storage::DBI/;
+use base qw/DBIx::Class::Storage::DBI/;
 use mro 'c3';
 
 use List::Util();
@@ -190,7 +190,10 @@ sub _select_args_to_query {
 
   # see if this is an ordered subquery
   my $attrs = $_[3];
-  if ( scalar $self->sql_maker->_order_by_chunks ($attrs->{order_by}) ) {
+  if ( scalar $self->_parse_order_by ($attrs->{order_by}) ) {
+    $self->throw_exception(
+      'An ordered subselect encountered - this is not safe! Please see "Ordered Subselects" in DBIx::Class::Storage::DBI::MSSQL
+    ') unless $attrs->{unsafe_subselect_ok};
     my $max = 2 ** 32;
     $sql =~ s/^ \s* SELECT \s/SELECT TOP $max /xi;
   }
@@ -305,6 +308,54 @@ $table_name ON>. Unfortunately this operation in MSSQL requires the
 C<db_ddladmin> privilege, which is normally not included in the standard
 write-permissions.
 
+=head2 Ordered Subselects
+
+If you attempted the following query (among many others) in Microsoft SQL
+Server
+
+ $rs->search ({}, {
+  prefetch => 'relation',
+  rows => 2,
+  offset => 3,
+ });
+
+You may be surprised to receive an exception. The reason for this is a quirk
+in the MSSQL engine itself, and sadly doesn't have a sensible workaround due
+to the way DBIC is built. DBIC can do truly wonderful things with the aid of
+subselects, and does so automatically when necessary. The list of situations
+when a subselect is necessary is long and still changes often, so it can not
+be exhaustively enumerated here. The general rule of thumb is a joined
+L<has_many|DBIx::Class::Relationship/has_many> relationship with limit/group
+applied to the left part of the join.
+
+In its "pursuit of standards" Microsft SQL Server goes to great lengths to
+forbid the use of ordered subselects. This breaks a very useful group of
+searches like "Give me things number 4 to 6 (ordered by name), and prefetch
+all their relations, no matter how many". While there is a hack which fools
+the syntax checker, the optimizer may B<still elect to break the subselect>.
+Testing has determined that while such breakage does occur (the test suite
+contains an explicit test which demonstrates the problem), it is relative
+rare. The benefits of ordered subselects are on the other hand too great to be
+outright disabled for MSSQL.
+
+Thus compromise between usability and perfection is the MSSQL-specific
+L<resultset attribute|DBIx::Class::ResultSet/ATTRIBUTES> C<unsafe_subselect_ok>.
+It is deliberately not possible to set this on the Storage level, as the user
+should inspect (and preferrably regression-test) the return of every such
+ResultSet individually. The example above would work if written like:
+
+ $rs->search ({}, {
+  unsafe_subselect_ok => 1,
+  prefetch => 'relation',
+  rows => 2,
+  offset => 3,
+ });
+
+If it is possible to rewrite the search() in a way that will avoid the need
+for this flag - you are urged to do so. If DBIC internals insist that an
+ordered subselect is necessary for an operation, and you believe there is a
+differnt/better way to get the same result - please file a bugreport.
+
 =head1 AUTHOR
 
 See L<DBIx::Class/CONTRIBUTORS>.