Session Options - SQL Server to Aurora PostgreSQL Migration Playbook
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Session Options

Feature compatibility Amazon SCT / Amazon DMS automation level Amazon SCT action code index Key differences


                              Two star feature compatibility

N/A

N/A

SET options are significantly different, except for transaction isolation control.

SQL Server Usage

Session options in SQL Server is a collection of run-time settings that control certain aspects of how the server handles data for individual sessions. A session is the period between a login event and a disconnect event or an exec sp_reset_connection command for connection pooling.

Each session may have multiple run scopes, which are all the statements before the GO keyword used in SQL Server Management Studio scripts, or any set of commands sent as a single run batch by a client application. Each run scope may contain additional sub-scopes. For example, scripts calling stored procedures or functions.

You can set the global session options, which all run scopes use by default, using the SET T-SQL command. Server code modules such as stored procedures and functions may have their own run context settings, which are saved along with the code to guarantee the validity of results.

Developers can explicitly use SET commands to change the default settings for any session or for an run scope within the session. Typically, client applications send explicit SET commands upon connection initiation.

You can view the metadata for current sessions using the sp_who_system stored procedure and the sysprocesses system table.

Note

To change the default setting for SQL Server Management Studio, choose Tools, Options, Query Execution, SQL Server, Advanced.

Syntax

Syntax for the SET command:

SET
Category Setting
Date and time    DATEFIRST | DATEFORMAT
Locking          DEADLOCK_PRIORITY | SET LOCK_TIMEOUT
Miscellaneous    CONCAT_NULL_YIELDS_NULL | CURSOR_CLOSE_ON_COMMIT | FIPS_FLAGGER | SET IDENTITY_INSERT | LANGUAGE | OFFSETS | QUOTED_IDENTIFIER
Query Execution  ARITHABORT | ARITHIGNORE | FMTONLY | NOCOUNT | NOEXEC | NUMERIC_ROUNDABORT | PARSEONLY | QUERY_GOVERNOR_COST_LIMIT | ROWCOUNT | TEXTSIZE
ANSI             ANSI_DEFAULTS | ANSI_NULL_DFLT_OFF | ANSI_NULL_DFLT_ON | ANSI_NULLS | ANSI_PADDING | ANSI_WARNINGS
Execution Stats  FORCEPLAN | SHOWPLAN_ALL | SHOWPLAN_TEXT | SHOWPLAN_XML | STATISTICS IO | STATISTICS XML | STATISTICS PROFILE | STATISTICS TIME
Transactions     IMPLICIT_TRANSACTIONS | REMOTE_PROC_TRANSACTIONS | TRANSACTION ISOLATION LEVEL | XACT_ABORT

For more information, see SET Statements (Transact-SQL) in the SQL Server documentation.

SET ROWCOUNT for DML Deprecated Setting

The SET ROWCOUNT for DML statements has been deprecated as of SQL Server 2008. For more information, see Deprecated Database Engine Features in SQL Server 2008 R2 in the SQL Server documentation.

For SSQL Server version 2008 R2 and lower, you could limit the number of rows affected by INSERT, UPDATE, and DELETE operations using SET ROWCOUNT. For example, it is a common practice in SQL Server to batch large DELETE or UPDATE operations to avoid transaction logging issues.

The following example loops and deletes rows where ForDelete is set to 1, but only 5000 rows at a time in separate transactions (assuming the loop isn’t within an explicit transaction).

SET ROWCOUNT 5000;
WHILE @@ROWCOUNT > 0
BEGIN
  DELETE FROM MyTable
  WHERE ForDelete = 1;
END

Starting from SQL Server 2012, SET ROWCOUNT is ignored for INSERT, UPDATE, and DELETE statements. You can achieve the same functionality using TOP. You can convert TOP to the Aurora PostgreSQL LIMIT.

For example, you can rewrite the preceding code as:

WHILE @@ROWCOUNT > 0
BEGIN
DELETE TOP (5000)
  FROM MyTable
  WHERE ForDelete = 1;
END

Amazon Schema Conversion Tool can convert this syntax automatically.

Examples

Use SET within a stored procedure.

CREATE PROCEDURE <ProcedureName>
AS
BEGIN
  <Some non-critical transaction code>
  SET TRANSACTION_ISOLATION_LEVEL SERIALIZABLE;
  SET XACT_ABORT ON;
  <Some critical transaction code>
END

Explicit SET commands affect their run scope and sub scopes. After the scope terminates and the procedure code exits, the calling scope resumes its original settings used before the calling the stored procedure.

For more information, see SET Statements (Transact-SQL) in the SQL Server documentation.

PostgreSQL Usage

Amazon Aurora PostgreSQL-Compatible Edition (Aurora PostgreSQL) supports hundreds of Server System Variables to control server behavior and the global and session levels.

PostgreSQL provides session-modifiable parameters that are configured using the SET SESSION command. Configuration of parameters using SET SESSION will only be applicable in the current session. To view the list of parameters that can be set with SET SESSION, you can query pg_settings:

SELECT * FROM pg_settings where context = 'user';

Examples of commonly used session parameters:

  • client_encoding configures the connected client character set.

  • force_parallel_mode forces use of parallel query for the session.

  • lock_timeout sets the maximum allowed duration of time to wait for a database lock to release.

  • search_path sets the schema search order for object names that aren’t schema-qualified.

  • transaction_isolation sets the current Transaction Isolation Level for the session.

You can view Aurora PostgreSQL variables using the PostgreSQL command line utility, Amazon Aurora database cluster parameters, Amazon Aurora database instance parameters, or SQL Server interface system variables.

Converting from SQL Server 2008 SET ROWCOUNT for DML operations

The use of SET ROWCOUNT for DML operations is deprecated as of SQL Server 2008 R2. Code that uses the SET ROWCOUNT syntax can’t be converted automatically.

You can either rewrite the code to use TOP before running Amazon SCT, or manually change it afterward.

Consider the example that is used to batch DELETE operations in SQL Server using TOP:

WHILE @@ROWCOUNT > 0
BEGIN
  DELETE TOP (5000)
  FROM MyTable
  WHERE ForDelete = 1;
END

You can rewrite the preceding example to use the Aurora PostgreSQL LIMIT clause:

WHILE row_count() > 0 LOOP
  DELETE FROM num_test
  WHERE ctid IN (
  SELECT ctid
  FROM num_test
  LIMIT 10)
END LOOP;

Examples

Change the time zone of the connected session.

SET SESSION DateStyle to POSTGRES, DMY;
SET

SELECT NOW();

now
Sat 09 Sep 11:03:43.597202 2017 UTC
(1 row)

SET SESSION DateStyle to ISO, MDY;
SET

SELECT NOW();

now
2017-09-09 11:04:01.3859+00
(1 row)

Summary

The following table summarizes commonly used SQL Server session options and their corresponding Aurora PostgreSQL system variables.

Category SQL Server Aurora PostgreSQL

Date and time

DATEFIRST

Use DOW in queries

Date and time

DATEFORMAT

DateStyle

Locking

LOCK_TIMEOUT

lock_timeout

Transactions

IMPLICIT_TRANSACTIONS

SET TRANSACTION

Transactions

TRANSACTION ISOLATION LEVEL

BEGIN TRANSACTION ISOLATION LEVEL

Query run

IDENTITY_INSERT

See Sequences and Identity.

Query run

LANGUAGE

lc_monetary, lc_numeric, or lc_time

Query run

QUOTED_IDENTIFIER

N/A

Query run

NOCOUNT

N/A and not needed

Run stats

SHOWPLAN_ALL, TEXT, XML, STATISTICS IO, PROFILE, and TIME

See Run Plans.

Miscellaneous

CONCAT_NULL_YIELDS_NULL

N/A

Miscellaneous

ROWCOUNT

Use LIMIT within SELECT.

For more information, see SET in the PostgreSQL documentation.