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

Summary of Delete Vs Truncate in SQL server

Taken from CodAffection article : Delete Vs Truncate in SQL Server

Answer 2

Here is a summary of some important differences between these sql commands:

sql truncate command:

1) It is a DDL (Data Definition Language) command, therefore commands such as COMMIT and ROLLBACK do not apply to this command (the exceptions here are PostgreSQL and MSSQL, whose implementation of the TRUNCATE command allows the command to be used in a transaction)

2) You cannot undo the operation of deleting records, it occurs automatically and is irreversible (except for the above exceptions - provided, however, that the operation is included in the TRANSACTION block and the session is not closed). In case of Oracle - Includes two implicit commits, one before and one after the statement is executed. Therefore, the command cannot be withdrawn while a runtime error will result in commit anyway

3) Deletes all records from the table, records cannot be limited to deletion. For Oracle, when the table is split per partition, individual partitions can be truncated (TRUNCATE) in isolation, making it possible to partially remove all data from the table

4) Frees up the space occupied by the data in the table (in the TABLESPACE - on disk). For Oracle - if you use the REUSE STORAGE clause, the data segments will not be rolled back, i.e. you will keep space from the deleted rows allocated to the table, which can be a bit more efficient if the table is to be reloaded with data. The high mark will be reset

5) TRUNCATE works much faster than DELETE

6) Oracle Flashback in the case of TRUNCATE prevents going back to pre-operative states

7) Oracle - TRUNCATE cannot be granted (GRANT) without using DROP ANY TABLE

8) The TRUNCATE operation makes unusable indexes usable again

9) TRUNCATE cannot be used when the enabled foreign key refers to another table, then you can:

• execute the command: DROP CONSTRAINT, then TRUNCATE, and then play it through CREATE CONSTRAINT or
• execute the command: SET FOREIGN_KEY_CHECKS = 0; then TRUNCATE, then: SET_FOREIGN_KEY_CHECKS = 1;

10) TRUNCATE requires an exclusive table lock, therefore, turning off exclusive table lock is a way to prevent TRUNCATE operation on the table

11) DML triggers do not fire after executing TRUNCATE (so be very careful in this case, you should not use TRUNCATE, if a delete trigger is defined in the table to perform an automatic table cleanup or a logon action after row deletion). On Oracle, DDL triggers are fired

12) Oracle - TRUNCATE cannot be used in the case of: database link 13) TRUNCATE does not return the number of records deleted

14) Transaction log - one log indicating page deallocation (removes data, releasing allocation of data pages used for storing table data and writes only page deallocations to the transaction log) - faster execution than DELETE. TRUNCATE only needs to adjust the pointer in the database to the table (High Water Mark) and the data is immediately deleted, therefore it uses less system resources and transaction logs

15) Performance (acquired lock) - table and page lock - does not degrade performance during execution

16) TRUNCATE cannot be used on tables involved in transactional replication or merge replication

sql delete command:

1) It is a DML (Data Manipulation Language) command, therefore the following commands are used for this command: COMMIT and ROLLBACK

2) You can undo the operation of removing records by using the ROLLBACK command

3) Deletes all or some records from the table, you can limit the records to be deleted by using the WHERE clause

4) Does not free the space occupied by the data in the table (in the TABLESPACE - on the disk)

5) DELETE works much slower than TRUNCATE

6) Oracle Flashback works for DELETE

7) Oracle - For DELETE, you can use the GRANT command

8) The DELETE operation does not make unusable indexes usable again

9) DELETE in case foreign key enabled refers to another table, can (or not) be applied depending on foreign key configuration (if not), please:

• execute the command: DROP CONSTRAINT, then TRUNCATE, and then play it through CREATE CONSTRAINT or
• execute the command: SET FOREIGN_KEY_CHECKS = 0; then TRUNCATE, then: SET_FOREIGN_KEY_CHECKS = 1;

10) DELETE requires a shared table lock

11) Triggers fire

12) DELETE can be used in the case of: database link

13) DELETE returns the number of records deleted

14) Transaction log - for each deleted record (deletes rows one at a time and records an entry in the transaction log for each deleted row) - slower execution than TRUNCATE. The table may still contain blank pages after executing the DELETE statement. DELETE needs to read records, check constraints, update block, update indexes, and generate redo / undo. All of this takes time, hence it takes time much longer than with TRUNCATE

15) Performance (acquired lock) - record lock - reduces performance during execution - each record in the table is locked for deletion

16) DELETE can be used on a table used in transactional replication or merge replication

Answer 3

DROP

The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.

TRUNCATE

TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUNCATE is faster and doesn't use as much undo space as a DELETE. Table level lock will be added when Truncating.

DELETE

The DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire. Row level lock will be added when deleting.

From: http://www.orafaq.com/faq/difference_between_truncate_delete_and_drop_commands

Answer 4

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.

---

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.

---

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.

---

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.

---

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.

---

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.

---

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

---

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.

---

Redo/Undo

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

---

Indexes

Oracle

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

---

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.

---

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.

---

Triggers

DML triggers do not fire on a truncate.

Oracle

DDL triggers are available.

---

Remote Execution

Oracle

Truncate cannot be issued over a database link.

---

Identity Columns

SQL*Server

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

---

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;

Answer 5

The difference between truncate and delete is listed below:

+----------------------------------------+----------------------------------------------+
|                Truncate                |                    Delete                    |
+----------------------------------------+----------------------------------------------+
| We can't Rollback after performing     | We can Rollback after delete.                |
| Truncate.                              |                                              |
|                                        |                                              |
| Example:                               | Example:                                     |
| BEGIN TRAN                             | BEGIN TRAN                                   |
| TRUNCATE TABLE tranTest                | DELETE FROM tranTest                         |
| SELECT * FROM tranTest                 | SELECT * FROM tranTest                       |
| ROLLBACK                               | ROLLBACK                                     |
| SELECT * FROM tranTest                 | SELECT * FROM tranTest                       |
+----------------------------------------+----------------------------------------------+
| Truncate reset identity of table.      | Delete does not reset identity of table.     |
+----------------------------------------+----------------------------------------------+
| It locks the entire table.             | It locks the table row.                      |
+----------------------------------------+----------------------------------------------+
| Its DDL(Data Definition Language)      | Its DML(Data Manipulation Language)          |
| command.                               | command.                                     |
+----------------------------------------+----------------------------------------------+
| We can't use WHERE clause with it.     | We can use WHERE to filter data to delete.   |
+----------------------------------------+----------------------------------------------+
| Trigger is not fired while truncate.   | Trigger is fired.                            |
+----------------------------------------+----------------------------------------------+
| Syntax :                               | Syntax :                                     |
| 1) TRUNCATE TABLE table_name           | 1) DELETE FROM table_name                    |
|                                        | 2) DELETE FROM table_name WHERE              |
|                                        |    example_column_id IN (1,2,3)              |
+----------------------------------------+----------------------------------------------+

Answer 6

Truncate command is used to re-initialize the table, it is a DDL command which delete all the rows of table.Whereas DELETE is a DML command which is used to delete row or set of rows according to some condition, if condition is not specified then this command will delete all the rows from the table.

Answer 7

One more difference specific to microsoft sql server is with delete you can use output statement to track what records have been deleted, e.g.:

delete from [SomeTable]
output deleted.Id, deleted.Name

You cannot do this with truncate.

Answer 8

DELETE

The DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire.

TRUNCATE

TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUCATE is faster and doesn't use as much undo space as a DELETE.

DROP

The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.

---

DROP and TRUNCATE are DDL commands, whereas DELETE is a DML command. Therefore DELETE operations can be rolled back (undone), while DROP and TRUNCATE operations cannot be rolled back.

From: http://www.orafaq.com/faq/difference_between_truncate_delete_and_drop_commands

Answer 9

DELETE statement can have a WHERE clause to delete specific records whereas TRUNCATE statement does not require any and wipes the entire table. Importantly, the DELETE statement logs the deleted date whereas the TRUNCATE statement does not.

Answer 10

TRUNCATE is the DDL statement whereas DELETE is a DML statement. Below are the differences between the two:

1. As TRUNCATE is a DDL (Data definition language) statement it does not require a commit to make the changes permanent. And this is the reason why rows deleted by truncate could not be rollbacked. On the other hand DELETE is a DML (Data manipulation language) statement hence requires explicit commit to make its effect permanent.

2. TRUNCATE always removes all the rows from a table, leaving the table empty and the table structure intact whereas DELETE may remove conditionally if the where clause is used.

3. The rows deleted by TRUNCATE TABLE statement cannot be restored and you can not specify the where clause in the TRUNCATE statement.

4. TRUNCATE statements does not fire triggers as opposed of on delete trigger on DELETE statement

Here is the very good link relevant to the topic.

Answer 11

With SQL Server or MySQL, if there is a PK with auto increment, truncate will reset the counter.

Answer 12

All good answers, to which I must add:

Since TRUNCATE TABLE is a DDL (Data Defination Language), not a DML (Data Manipulation Langauge) command, the Delete Triggers do not run.

Answer 13

Here is my detailed answer on the difference between DELETE and TRUNCATE in SQL Server

• Remove Data : First thing first, both can be used to remove the rows from table.
But a DELETE can be used to remove the rows not only from a Table but also from a VIEW or the result of an OPENROWSET or OPENQUERY subject to provider capabilities.

• FROM Clause : With DELETE you can also delete rows from one table/view/rowset_function_limited based on rows from another table by using another FROM clause. In that FROM clause you can also write normal JOIN conditions. Actually you can create a DELETE statement from a SELECT statement that doesn’t contain any aggregate functions by replacing SELECT with DELETE and removing column names.
With TRUNCATE you can’t do that.

• WHERE : A TRUNCATE cannot have WHERE Conditions, but a DELETE can. That means with TRUNCATE you can’t delete a specific row or specific group of rows. TRUNCATE TABLE is similar to the DELETE statement with no WHERE clause.

• Performance : TRUNCATE TABLE is faster and uses fewer system and transaction log resources. And one of the reason is locks used by either statements. The DELETE statement is executed using a row lock, each row in the table is locked for deletion. TRUNCATE TABLE always locks the table and page but not each row.

• Transaction log : DELETE statement removes rows one at a time and makes individual entries in the transaction log for each row.
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 log.

• Pages : After a DELETE statement is executed, the table can still contain empty pages. TRUNCATE removes the data by deallocating the data pages used to store the table data.

• Trigger : TRUNCATE does not activate the delete triggers on the table. So you must be very careful while using TRUNCATE. One should never use a TRUNCATE if delete Trigger is defined on the table to do some automatic cleanup or logging action when rows are deleted.

• Identity Column : With TRUNCATE if the table contains an identity column, the counter for that column is reset to the seed value defined for the column. If no seed was defined, the default value 1 is used. DELETE doesn’t reset the identity counter. So if you want to retain the identity counter, use DELETE instead.

• Replication : DELETE can be used against table used in transactional replication or merge replication.
While TRUNCATE cannot be used against the tables involved in transactional replication or merge replication.

• Rollback : DELETE statement can be rolled back.
TRUNCATE can also be rolled back provided it is enclosed in a TRANSACTION block and session is not closed. Once session is closed you won't be able to Rollback TRUNCATE.

• Restrictions : The DELETE statement may fail if it violates a trigger or tries to remove a row referenced by data in another table with a FOREIGN KEY constraint. If the DELETE removes multiple rows, and any one of the removed rows violates a trigger or constraint, the statement is canceled, an error is returned, and no rows are removed.
And if DELETE is used against View, that View must be an Updatable view. TRUNCATE cannot be used against the table used in Indexed view.
TRUNCATE cannot be used against the table referenced by a FOREIGN KEY constraint, unless a table that has a foreign key that references itself.

Answer 14

One further difference of the two operations is that if the table contains an identity column, the counter for that column is reset 1 (or to the seed value defined for the column) under TRUNCATE. DELETE does not have this affect.

Answer 15

By issuing a TRUNCATE TABLE statement, you are instructing SQL Server to delete every record within a table, without any logging or transaction processing taking place.

Answer 16

Truncate and Delete in SQL are two commands which is used to remove or delete data from table. Though quite basic in nature both Sql commands can create lot of trouble until you are familiar with details before using it. An Incorrect choice of command can result is either very slow process or can even blew up log segment, if too much data needs to be removed and log segment is not enough. That's why it's critical to know when to use truncate and delete command in SQL but before using these you should be aware of the Differences between Truncate and Delete, and based upon them, we should be able to find out when DELETE is better option for removing data or TRUNCATE should be used to purge tables.

Refer check click here

Answer 17

TRUNCATE can be rolled back if wrapped in a transaction.

Please see the two references below and test yourself:-

http://blog.sqlauthority.com/2007/12/26/sql-server-truncate-cant-be-rolled-back-using-log-files-after-transaction-session-is-closed/

http://sqlblog.com/blogs/kalen_delaney/archive/2010/10/12/tsql-tuesday-11-rolling-back-truncate-table.aspx

The TRUNCATE vs. DELETE is one of the infamous questions during SQL interviews. Just make sure you explain it properly to the Interviewer or it might cost you the job. The problem is that not many are aware so most likely they will consider the answer as wrong if you tell them that YES Truncate can be rolled back.

Answer 18

If accidentally you removed all the data from table using Delete/Truncate. You can rollback committed transaction. Restore the last backup and run transaction log till the time when Delete/Truncate is about to happen.

The related information below is from a blog post:

While working on database, we are using Delete and Truncate without knowing the differences between them. In this article we will discuss the difference between Delete and Truncate in Sql.

Delete:

• Delete is a DML command.
• Delete statement is executed using a row lock,each row in the table is locked for deletion.
• We can specify filters in where clause.
• It deletes specified data if where condition exists.
• Delete activities a trigger because the operation are logged individually.
• Slower than Truncate because it Keeps logs

Truncate

• Truncate is a DDL command.
• Truncate table always lock the table and page but not each row.As it removes all the data.
• Cannot use Where condition.
• It Removes all the data.
• Truncate table cannot activate a trigger because the operation does not log individual row deletions.
• Faster in performance wise, because it doesn't keep any logs.

Note: Delete and Truncate both can be rolled back when used with Transaction. If Transaction is done, means committed then we can not rollback Truncate command, but we can still rollback Delete command from Log files, as delete write records them in Log file in case it is needed to rollback in future from log files.

If you have a Foreign key constraint referring to the table you are trying to truncate, this won't work even if the referring table has no data in it. This is because the foreign key checking is done with DDL rather than DML. This can be got around by temporarily disabling the foreign key constraint(s) to the table.

Delete table is a logged operation. So the deletion of each row gets logged in the transaction log, which makes it slow. Truncate table also deletes all the rows in a table, but it won't log the deletion of each row instead it logs the deallocation of the data pages of the table, which makes it faster.

~ If accidentally you removed all the data from table using Delete/Truncate. You can rollback committed transaction. Restore the last backup and run transaction log till the time when Delete/Truncate is about to happen.

Answer 19

Truncate can also be Rollbacked here the exapmle

begin Tran
delete from  Employee

select * from Employee
Rollback
select * from Employee

Answer 20

It is not that truncate does not log anything in SQL Server. truncate does not log any information but it log the deallocation of data page for the table on which you fired TRUNCATE.

and truncated record can be rollback if we define transaction at beginning and we can recover the truncated record after rollback it. But can not recover truncated records from the transaction log backup after committed truncated transaction.

Answer 21

DELETE

DELETE is a DML command
DELETE you can rollback
Delete = Only Delete- so it can be rolled back
In DELETE you can write conditions using WHERE clause
Syntax – Delete from [Table] where [Condition]

TRUNCATE

TRUNCATE is a DDL command
You can't rollback in TRUNCATE, TRUNCATE removes the record permanently
Truncate = Delete+Commit -so we can't roll back
You can't use conditions(WHERE clause) in TRUNCATE
Syntax – Truncate table [Table]

For more details visit

http://www.zilckh.com/what-is-the-difference-between-truncate-and-delete/

Answer 22

DELETE Statement: This command deletes only the rows from the table based on the condition given in the where clause or deletes all the rows from the table if no condition is specified. But it does not free the space containing the table.

The Syntax of a SQL DELETE statement is:

DELETE FROM table_name [WHERE condition];

TRUNCATE statement: This command is used to delete all the rows from the table and free the space containing the table.

Answer 23

In SQL Server 2005 I believe that you can rollback a truncate

Answer 24

A small correction to the original answer - delete also generates significant amounts of redo (as undo is itself protected by redo). This can be seen from autotrace output:

SQL> delete from t1;

10918 rows deleted.

Elapsed: 00:00:00.58

Execution Plan
----------------------------------------------------------
   0      DELETE STATEMENT Optimizer=FIRST_ROWS (Cost=43 Card=1)
   1    0   DELETE OF 'T1'
   2    1     TABLE ACCESS (FULL) OF 'T1' (TABLE) (Cost=43 Card=1)




Statistics
----------------------------------------------------------
         30  recursive calls
      12118  db block gets
        213  consistent gets
        142  physical reads
    3975328  redo size
        441  bytes sent via SQL*Net to client
        537  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
      10918  rows processed

Answer 25

Yes, DELETE is slower, TRUNCATE is faster. Why?

DELETE must read the records, check constraints, update the block, update indexes, and generate redo/undo. All of that takes time.

TRUNCATE simply adjusts a pointer in the database for the table (the High Water Mark) and poof! the data is gone.

This is Oracle specific, AFAIK.

Answer 26

TRUNCATE is fast, DELETE is slow.

Although, TRUNCATE has no accountability.

Answer 27

I'd comment on matthieu's post, but I don't have the rep yet...

In MySQL, the auto increment counter gets reset with truncate, but not with delete.

Answer 28

Can't do DDL over a dblink.

Answer 29

A big reason it is handy, is when you need to refresh the data in a multi-million row table, but don't want to rebuild it. "Delete *" would take forever, whereas the perfomance impact of Truncate would be negligible.

Answer 30

"Truncate doesn't log anything" is correct. I'd go further:

Truncate is not executed in the context of a transaction.

The speed advantage of truncate over delete should be obvious. That advantage ranges from trivial to enormous, depending on your situation.

However, I've seen truncate unintentionally break referential integrity, and violate other constraints. The power that you gain by modifying data outside a transaction has to be balanced against the responsibility that you inherit when you walk the tightrope without a net.