What are the sql specifications?

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See the Wechat post from itpub, move it, it should not belong to optimization, it should belong to the basic sql specification:

The 1.ORACLE parser processes the table names in the from clause from right to left, so the table written later (the underlying table driving table) will be processed first, choosing the one with the least number of records as the driving table

two。 Table joins are written before other where conditions. You can filter out the conditions most recorded by big data and write them at the end of the where sentence.

Avoid using * in 3.select sentences

4. Set ARRAYSIZE to increase the amount of data retrieved per database access. 200 is recommended.

Show arraysize

Set arraysize 200

Permanent: this parameter can be saved to a glogin.sql or login.sql file, so it can take effect permanently without having to specify it in set every time.

5. Use the decode function to reduce processing time

6. Easy to integrate, unrelated database access

7. Delete duplicate records:

Delete from emp a where a.rowid > (select min (x.rowid))

8.truncate replaces delete

9. Be sure to remember commit, multi-commit:

Release the data information of the undo segment for recovery

Release lock

Free up redo log buffer space

10. Improve sql efficiency through internal functions

11. Use the alias of the table

twelve。 Replace in,not exists with exists instead of not in

13. Be good at using indexes

14.sql statement case specification, in uppercase

15. Avoid using not and calculations on indexes

16. Replace > with > =

17. Replace index columns with union replace or with union in the or:where sentence (insist on using or, write the index column with the least number of records at the front)

18. Avoid resource-consuming operations: distinct,union,minus,intersect,order by

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