Wednesday, 17 October 2012

Delete vs Truncate in SQL

TRUNCATE is a DDL commandDELETE is a DML command
TRUNCATE TABLE always locks the table and page but not each rowDELETE statement is executed using a row lock,                                             each row in the table is locked for deletion
Cannot use Where ConditionWe can specify filters in where clause
It Removes all the dataIt deletes specified data if where condition exists.
TRUNCATE TABLE cannot activate a trigger because the operation does not log individual row deletions.Delete activates a trigger because the operation                                are logged individually.
Faster in performance wise, because it is minimally logged in transaction log.Slower than truncate because, it maintain logs for every record
 Drop all object’s statistics and marks like High Water Mark free extents and leave the object really empty with the first extent. zero pages are left in the tablekeeps object’s statistics and all allocated space. After a                       DELETE statement is executed,the table can still contain empty pages.
TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction loThe DELETE statement removes rows one at a time                       and records an entry in the transaction log for each deleted row
If the table contains an identity column, the counter for that column is reset to the seed value that is defined for the columnDELETE retain the identity
Restrictions on using Truncate Statement
1. Are referenced by a FOREIGN KEY constraint.
2. Participate in an indexed view.
3. Are published by using transactional replication or merge replication.
Delete works at row level, thus row level constrains apply

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