Thursday, 7 June 2012

Table Variable V/S Temporary Table

Difference between Table Variable and temporary table

Feature Table Variables Temporary Tables
Scope Current batch Current session, nested stored procedures. Global: all sessions.
Usage UDFs, Stored Procedures, Triggers, Batches. Stored Procedures, Triggers, Batches.
Creation DECLARE statement only.

CREATE TABLE statement.

SELECT INTO statement.

Table name Maximum 128 characters. Maximum 116 characters.
Column data types

Can use user-defined data types.

Can use XML collections.

User-defined data types and XML collections must be in tempdb to use.
Collation String columns inherit collation from current database. String columns inherit collation from tempdb database.
Indexes Can only have indexes that are automatically created with PRIMARY KEY & UNIQUE constraints as part of the DECLARE statement. Indexes can be added after the table has been created.
Constraints PRIMARY KEY, UNIQUE, NULL, CHECK, but they must be incorporated with the creation of the table in the DECLARE statement. FOREIGN KEY not allowed. PRIMARY KEY, UNIQUE, NULL, CHECK. Can be part of the CREATE TABLE statement, or can be added after the table has been created. FOREIGN KEY not allowed.
Post-creation DDL (indexes, columns) Statements are not allowed. Statements are allowed.
Data insertion INSERT statement (SQL 2000: cannot use INSERT/EXEC).

INSERT statement, including INSERT/EXEC.

SELECT INTO statement.

Insert explicit values into identity columns (SET IDENTITY_INSERT). The SET IDENTITY_INSERT statement is not supported. The SET IDENTITY_INSERT statement is supported.
Truncate table Not allowed. Allowed.
Destruction Automatically at the end of the batch. Explicitly with DROP TABLE statement. Automatically when session ends. (Global: also when other sessions have no statements using table.)
Transactions Last only for length of update against the table variable. Uses less than temporary tables. Last for the length of the transaction. Uses more than table variables.
Stored procedure recompilations Not applicable. Creating temp table and data inserts cause procedure recompilations.
Rollbacks Not affected (Data not rolled back). Affected (Data is rolled back).
Statistics Optimizer cannot create any statistics on columns, so it treats table variable has having 1 record when creating execution plans. Optimizer can create statistics on columns. Uses actual row count for generation execution plan.
Pass to stored procedures SQL 2008 only, with predefined user-defined table type. Not allowed to pass, but they are still in scope to nested procedures.
Explicitly named objects (indexes, constraints). Not allowed. Allowed, but be aware of multi-user issues.
Dynamic SQL Must declare table variable inside the dynamic SQL. Can use temporary tables created prior to calling the dynamic sql.

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