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DBIx::Class::Storage::User:Contributed PerlDBIx::Class::Storage::DBI::MSSQL(3)

NAME
       DBIx::Class::Storage::DBI::MSSQL	- Base Class for Microsoft SQL Server
       support in DBIx::Class

SYNOPSIS
       This is the base	class for Microsoft SQL	Server support,	used by
       DBIx::Class::Storage::DBI::ODBC::Microsoft_SQL_Server and
       DBIx::Class::Storage::DBI::Sybase::Microsoft_SQL_Server.

IMPLEMENTATION NOTES
   IDENTITY information
       Microsoft SQL Server supports three methods of retrieving the IDENTITY
       value for inserted row: IDENT_CURRENT, @@IDENTITY, and
       SCOPE_IDENTITY().  SCOPE_IDENTITY is used here because it is the
       safest.	However, it must be called is the same execute statement, not
       just the	same connection.

       So, this	implementation appends a SELECT	SCOPE_IDENTITY() statement
       onto each INSERT	to accommodate that requirement.

       "SELECT @@IDENTITY" can also be used by issuing:

	 $self->_identity_method('@@identity');

       it will only be used if SCOPE_IDENTITY()	fails.

       This is more dangerous, as inserting into a table with an on insert
       trigger that inserts into another table with an identity	will give
       erroneous results on recent versions of SQL Server.

   identity insert
       Be aware	that we	have tried to make things as simple as possible	for
       our users.  For MSSQL that means	that when a user tries to create a
       row, while supplying an explicit	value for an autoincrementing column,
       we will try to issue the	appropriate database call to make this
       possible, namely	"SET IDENTITY_INSERT $table_name ON". Unfortunately
       this operation in MSSQL requires	the "db_ddladmin" privilege, which is
       normally	not included in	the standard write-permissions.

   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 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 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
       resultset attribute "unsafe_subselect_ok".  It is deliberately not
       possible	to set this on the Storage level, as the user should inspect
       (and preferably 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 different/better way to get the same result -	please
       file a bugreport.

FURTHER	QUESTIONS?
       Check the list of additional DBIC resources.

COPYRIGHT AND LICENSE
       This module is free software copyright by the DBIx::Class (DBIC)
       authors.	You can	redistribute it	and/or modify it under the same	terms
       as the DBIx::Class library.

perl v5.32.0			  2020-03-2DBIx::Class::Storage::DBI::MSSQL(3)

NAME | SYNOPSIS | IMPLEMENTATION NOTES | FURTHER QUESTIONS? | COPYRIGHT AND LICENSE

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