Close the book on mssql ordered subqueries

diff --git a/lib/DBIx/Class/Storage/DBI/MSSQL.pm b/lib/DBIx/Class/Storage/DBI/MSSQL.pm
index 178a007..ea1269a 100644 (file)
--- a/lib/DBIx/Class/Storage/DBI/MSSQL.pm
+++ b/lib/DBIx/Class/Storage/DBI/MSSQL.pm
@@ -191,6 +191,9 @@ 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}) ) {
+    $self->throw_exception(
+      'An ordered subquery encountered. Please see "Ordered Subqueries" in DBIx::Class::Storage::DBI::MSSQL
+    ') unless $attrs->{unsafe_subquery};
     my $max = 2 ** 32;
     $sql =~ s/^ \s* SELECT \s/SELECT TOP $max /xi;
   }
@@ -305,6 +308,45 @@ $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 Subqueries
+
+ # this is deemed unsafe and throws under MSSQL
+ $rs->search ({}, {
+  prefetch => 'relation',
+  rows => 2,
+  offset => 3,
+ });
+
+ # however this should work (but please check what comes back from the db)
+ $rs->search ({}, {
+  unsafe_subquery => 1,
+  prefetch => 'relation',
+  rows => 2,
+  offset => 3,
+ });
+
+DBIC can do truly wonderful things with the aid of subqueries, and does so
+automatically when necessary. Especially useful are ordered subqueries,
+which allow things like "Give me things number 4 to 6 (ordered by name), and
+prefetch all their relationss, no matter how many". In its pursuit of standards
+Microsft SQL Server goes to great lengths to forbid the use of ordered
+subqueries. While there is a hack which fools the syntax checker, the optimizer
+may B<still elect to break the subquery>. 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
+subqueries 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_subquery>.
+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.
+
+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 subquery is necessary for an operation, and you believe there is a
+differnt way to express the query - please file a bugreport.
+
 =head1 AUTHOR
 
 See L<DBIx::Class/CONTRIBUTORS>.

diff --git a/t/746mssql.t b/t/746mssql.t
index c32797a..f73f5ff 100644 (file)
--- a/t/746mssql.t
+++ b/t/746mssql.t
@@ -255,9 +255,14 @@ lives_ok ( sub {
   ]);
 }, 'populate without PKs supplied ok' );
 
-# make sure ordered subselects work
+# plain ordered subqueries throw
+throws_ok (sub {
+  $schema->resultset('Owners')->search ({}, { order_by => 'name' })->as_query
+}, qr/ordered subquery encountered/, 'Ordered Subquery detection throws ok');
+
+# make sure ordered subselects *somewhat* work
 {
-  my $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3 });
+  my $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3, unsafe_subquery => 1 });
 
   my $al = $owners->current_source_alias;
   my $sealed_owners = $owners->result_source->resultset->search (
@@ -277,29 +282,13 @@ lives_ok ( sub {
     [ map { $_->name } ($owners->all) ],
     'Sort preserved from within a subquery',
   );
-
-
-  my $corelated_owners = $owners->result_source->resultset->search (
-    {
-      id => { -in => $owners->get_column('id')->as_query },
-    },
-    {
-      order_by => 'name'
-    },
-  );
-
-  is_deeply (
-    [ map { $_->name } ($corelated_owners->all) ],
-    [ map { $_->name } ($owners->all) ],
-    'Sort preserved from within a corelated subquery',
-  );
 }
 
 TODO: {
   local $TODO = "This porbably will never work, but it isn't critical either afaik";
 
   my $book_owner_ids = $schema->resultset ('BooksInLibrary')
-                               ->search ({}, { join => 'owner', distinct => 1, order_by => 'owner.name' })
+                               ->search ({}, { join => 'owner', distinct => 1, order_by => 'owner.name', unsafe_subquery => 1 })
                                 ->get_column ('owner');
 
   my $book_owners = $schema->resultset ('Owners')->search ({
@@ -313,6 +302,27 @@ TODO: {
   );
 }
 
+# This is known not to work - thus the negative test
+{
+  my $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3, unsafe_subquery => 1 });
+  my $corelated_owners = $owners->result_source->resultset->search (
+    {
+      id => { -in => $owners->get_column('id')->as_query },
+    },
+    {
+      order_by => 'name' #reorder because of what is shown above
+    },
+  );
+
+  cmp_ok (
+    join ("\x00", map { $_->name } ($corelated_owners->all) ),
+      'ne',
+    join ("\x00", map { $_->name } ($owners->all) ),
+    'Sadly sort not preserved from within a corelated subquery',
+  );
+}
+
+
 # make sure right-join-side single-prefetch ordering limit works
 {
   my $rs = $schema->resultset ('BooksInLibrary')->search (
@@ -334,7 +344,7 @@ TODO: {
     'Rows were properly ordered'
   );
 
-  my $limited_rs = $rs->search ({}, {rows => 7, offset => 2});
+  my $limited_rs = $rs->search ({}, {rows => 7, offset => 2, unsafe_subquery => 1});
   is ($limited_rs->count, 6, 'Correct count of limited right-sorted joined resultset');
   is ($limited_rs->count_rs->next, 6, 'Correct count_rs of limited right-sorted joined resultset');
 
@@ -380,6 +390,7 @@ $schema->storage->_sql_maker->{name_sep} = '.';
       prefetch => 'books',
       order_by => { -asc => \['name + ?', [ test => 'xxx' ]] }, # test bindvar propagation
       rows     => 3,  # 8 results total
+      unsafe_subquery => 1,
     },
   );
 
@@ -408,6 +419,7 @@ $schema->storage->_sql_maker->{name_sep} = '.';
       prefetch => 'owner',
       rows     => 2,  # 3 results total
       order_by => { -desc => 'owner' },
+      unsafe_subquery => 1,
     },
   );