What's the difference between TRUNCATE and DELETE in SQL

Question

What's the difference between TRUNCATE and DELETE in SQL?

If your answer is platform specific, please indicate that.

Answer 1

Here's a list of differences. I've highlighted Oracle-specific features, and hopefully the community can add in other vendors' specific difference also. Differences that are common to most vendors can go directly below the headings, with differences highlighted below.

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General Overview

If you want to quickly delete all of the rows from a table, and you're really sure that you want to do it, and you do not have foreign keys against the tables, then a TRUNCATE is probably going to be faster than a DELETE.

Various system-specific issues have to be considered, as detailed below.

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Statement type

Delete is DML, Truncate is DDL (What is DDL and DML?)

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Commit and Rollback

Variable by vendor

SQL*Server

Truncate can be rolled back.

PostgreSQL

Truncate can be rolled back.

Oracle

Because a TRUNCATE is DDL it involves two commits, one before and one after the statement execution. Truncate can therefore not be rolled back, and a failure in the truncate process will have issued a commit anyway.

However, see Flashback below.

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Space reclamation

Delete does not recover space, Truncate recovers space

Oracle

If you use the REUSE STORAGE clause then the data segments are not de-allocated, which can be marginally more efficient if the table is to be reloaded with data. The high water mark is reset.

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Row scope

Delete can be used to remove all rows or only a subset of rows. Truncate removes all rows.

Oracle

When a table is partitioned, the individual partitions can be truncated in isolation, thus a partial removal of all the table's data is possible.

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Object types

Delete can be applied to tables and tables inside a cluster. Truncate applies only to tables or the entire cluster. (May be Oracle specific)

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Data Object Identity

Oracle

Delete does not affect the data object id, but truncate assigns a new data object id unless there has never been an insert against the table since its creation Even a single insert that is rolled back will cause a new data object id to be assigned upon truncation.

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Flashback (Oracle)

Flashback works across deletes, but a truncate prevents flashback to states prior to the operation.

However, from 11gR2 the FLASHBACK ARCHIVE feature allows this, except in Express Edition

Use of FLASHBACK in Oracle http://docs.oracle.com/cd/E11882_01/appdev.112/e41502/adfns_flashback.htm#ADFNS638

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Privileges

Variable

Oracle

Delete can be granted on a table to another user or role, but truncate cannot be without using a DROP ANY TABLE grant.

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Redo/Undo

Delete generates a small amount of redo and a large amount of undo. Truncate generates a negligible amount of each.

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Indexes

Oracle

A truncate operation renders unusable indexes usable again. Delete does not.

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Foreign Keys

A truncate cannot be applied when an enabled foreign key references the table. Treatment with delete depends on the configuration of the foreign keys.

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Table Locking

Oracle

Truncate requires an exclusive table lock, delete requires a shared table lock. Hence disabling table locks is a way of preventing truncate operations on a table.

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Triggers

DML triggers do not fire on a truncate.

Oracle

DDL triggers are available.

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Remote Execution

Oracle

Truncate cannot be issued over a database link.

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Identity Columns

SQL*Server

Truncate resets the sequence for IDENTITY column types, delete does not.

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Result set

In most implementations, a DELETE statement can return to the client the rows that were deleted.

e.g. in an Oracle PL/SQL subprogram you could:

DELETE FROM employees_temp WHERE       employee_id = 299  RETURNING   first_name,             last_name INTO        emp_first_name,             emp_last_name;