In short, Truncate removes all rows from a table, leaving the table empty and the table structure intact. What do they mean leaving table structure intact? Do they mean Values () gets deleted and the create table(..., ..., ...) remains as it is? Delete does same thing, does not it?
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This very same question came up a couple of days ago:
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The differences basically boil down to:
TRUNCATE is much, much faster, but...
TRUNCATE does not get logged, and so cannot be rolled back, as you can if you DELETE within a transaction
Delete is a DML statement (Data Manipulation Language) and Truncate is a DDL statement (Data Definition Language).
Both statements delete data from a table without affecting the structure of the table.
You can specify a where clause with a delete statement to selectively remove rows from the table, but truncate removes ALL data from the table. Delete will log the rows that are deleted so that they can be rolled back (recovered) if required. After the transaction it may still be possible to recover the deleted data using a flashback query. This is due to deleted rows being written to the redo log.
Truncate does NOT log the rows being removed in the redo log therefore there is no way to roll it back or recover the removed data using flashback. Not only is truncate permanent, because it is DDL it will issue an implicit commit and end the current transaction. This means if you issue a truncate when other changes (insert, update delete) haven't been commited, the truncate statement will commit them.
Truncate is much faster than delete (depending on the number of rows being removed). This is because it simply adjusts the "pointers" in the table to indicate there is no longer any data in the table, whereas delete removes each identified row.
There is a big difference, at least in Oracle. TRUNCATE resets the high water mark, DELETE does not.
INSERT a million rows to make a 100mb table.
DELETE from table
SELECT * will perform like mud, full table scan over 100mb of empty blocks due to the high water mark
INSERT a million rows.
TRUNCATE table
SELECT * = fast, immediate return, no scanning of extents required, high water mark was reset by TRUNCATE
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by: angelIIIPosted on 2009-06-15 at 06:48:24ID: 24628727
the only real difference between TRUNCATE and DELETE is that TRUNCATE does not log all the deleted values in the transaction log.
it also requires a different level of permissions, and is not possible when you have foreign keys pointing to the table.
>What do they mean leaving table structure intact?
truncate just removes the data, and does not modify the table structure in any way (the same as for delete)