From: Peter Rabbitson Date: Sun, 10 Jan 2010 09:07:10 +0000 (+0000) Subject: Rename subquery to subselect and rewrite POD (per castaway) X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=d74f2da92c4cc45af8154f7925e5a8c2e1a2a30c;p=dbsrgits%2FDBIx-Class-Historic.git Rename subquery to subselect and rewrite POD (per castaway) --- diff --git a/lib/DBIx/Class/Storage/DBI/MSSQL.pm b/lib/DBIx/Class/Storage/DBI/MSSQL.pm index 0331180..1893c66 100644 --- a/lib/DBIx/Class/Storage/DBI/MSSQL.pm +++ b/lib/DBIx/Class/Storage/DBI/MSSQL.pm @@ -192,8 +192,8 @@ sub _select_args_to_query { 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}; + 'An ordered subselect encountered - this is not safe! Please see "Ordered Subselects" in DBIx::Class::Storage::DBI::MSSQL + ') unless $attrs->{unsafe_subselect}; my $max = 2 ** 32; $sql =~ s/^ \s* SELECT \s/SELECT TOP $max /xi; } @@ -308,43 +308,52 @@ $table_name ON>. Unfortunately this operation in MSSQL requires the C privilege, which is normally not included in the standard write-permissions. -=head2 Ordered Subqueries +=head2 Ordered Subselects - # this is deemed unsafe and throws under MSSQL - $rs->search ({}, { - prefetch => 'relation', - rows => 2, - offset => 3, - }); +If you attempted the following query (among many others) in Microsoft SQL +Server - # 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 searches like "Give me things number 4 to 6 (ordered by name), and -prefetch all their relations, 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. 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. +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 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. +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 C. +L C. 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. +ResultSet individually. The example above would work if written like: + + $rs->search ({}, { + unsafe_subselect => 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 subquery is necessary for an operation, and you believe there is a +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 diff --git a/t/746mssql.t b/t/746mssql.t index a4720c8..e70a3b8 100644 --- a/t/746mssql.t +++ b/t/746mssql.t @@ -258,11 +258,11 @@ lives_ok ( sub { # 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'); +}, qr/ordered subselect encountered/, 'Ordered Subselect detection throws ok'); # make sure ordered subselects *somewhat* work { - my $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3, unsafe_subquery => 1 }); + my $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3, unsafe_subselect => 1 }); my $al = $owners->current_source_alias; my $sealed_owners = $owners->result_source->resultset->search ( @@ -288,7 +288,7 @@ 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', unsafe_subquery => 1 }) + ->search ({}, { join => 'owner', distinct => 1, order_by => 'owner.name', unsafe_subselect => 1 }) ->get_column ('owner'); my $book_owners = $schema->resultset ('Owners')->search ({ @@ -304,7 +304,7 @@ 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 $owners = $schema->resultset ('Owners')->search ({}, { order_by => 'name', offset => 2, rows => 3, unsafe_subselect => 1 }); my $corelated_owners = $owners->result_source->resultset->search ( { id => { -in => $owners->get_column('id')->as_query }, @@ -351,7 +351,7 @@ TODO: { 'Rows were properly ordered' ); - my $limited_rs = $rs->search ({}, {rows => 7, offset => 2, unsafe_subquery => 1}); + my $limited_rs = $rs->search ({}, {rows => 7, offset => 2, unsafe_subselect => 1}); is ($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'); @@ -397,7 +397,7 @@ $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, + unsafe_subselect => 1, }, ); @@ -426,7 +426,7 @@ $schema->storage->_sql_maker->{name_sep} = '.'; prefetch => 'owner', rows => 2, # 3 results total order_by => { -desc => 'owner' }, - unsafe_subquery => 1, + unsafe_subselect => 1, }, );