Explain the difference between Oracle DELETE and TRUNCATE

1. Grammar

delete from table aa truncate table aa code>

Second, the difference

Conditions can be written after
• delete from, but not truncate.
• delete from records are deleted one by one, and each deleted line of records will be entered into the log, and truncate deletes the entire page at one time, so onlyRecord page release, in short , delete from update log, truncate basically not, the transaction log space used is less.
• delete from will leave an empty page after deleting an empty table, and truncate will not leave any pages in the table.
• When a DELETE statement is executed with row locks, the rows in the table are locked for deletion.truncate always locks tables and pages, not individual rows.
• If there is an auto-increment id column generated by identity, after delete from it still increases from the last number, that is, the seed remains unchanged, and truncate, the seed will be restored to its original state.
• truncate does not fire the trigger for delete because the truncate action does not record individual row deletions.

3. Summary

• truncate and delete only delete the data, not the structure (definition) of the table;
The
• drop statement will drop constraints (constrain), triggers (trigger), indexes (index); Stored procedures/functions that depend on this table will remain, but become invalid status.
• The delete statement is dml, this operation will be placed in rollback segement, it will take effect after the transaction is committed;If there is a corresponding trigger, it will be triggered when executed; truncate, drop is ddl, the operation is immediateEffective, the original data will not be placed in the rollback segment, cannot be rolled back, and the operation will not trigger trigger.
The
• delete statement does not affect the extent occupied by the table, and the high watermark remains in its original position; obviously the drop statementFree all the space occupied by the table.The truncate statement by default frees space to minextentsextents,unless reuse storage; truncate will reset the high watermark (back to the beginning).
• Speed, in general: drop>truncate>delete.
• Security: Be careful with drop and truncate, especially if there is no backup, otherwise it will be too late to cry.
• In use, if you want to delete some data rows, use delete, and pay attention to the where clause.The rollback segment should be large enough; want to drop the table.Of course with drop; delete all data if you want to keep the table.If it has nothing to do with the transaction, use truncate.If it is related to a transaction, or if you want to trigger trigger, use delete; if it is to sort out the fragments inside the table, you can use truncate to keep up with reuse stroage, and then re-import/insert the data.