Tutorial :How to efficiently delete rows from a Postgresql 8.1 table?



Question:

I'm working on a PostgreSQL 8.1 SQL script which needs to delete a large number of rows from a table.

Let's say the table I need to delete from is Employees (~260K rows). It has primary key named id.

The rows I need to delete from this table are stored in a separate temporary table called EmployeesToDelete (~10K records) with a foreign key reference to Employees.id called employee_id.

Is there an efficient way to do this?

At first, I thought of the following:

DELETE  FROM    Employees  WHERE   id IN          (          SELECT  employee_id          FROM    EmployeesToDelete          )  

But I heard that using the "IN" clause and subqueries can be inefficient, especially with larger tables.

I've looked at the PostgreSQL 8.1 documentation, and there's mention of DELETE FROM ... USING but it doesn't have examples so I'm not sure how to use it.

I'm wondering if the following works and is more efficient?

DELETE  FROM    Employees  USING   Employees e  INNER JOIN          EmployeesToDelete ed  ON      e.id = ed.employee_id  

Your comments are greatly appreciated.

Edit: I ran EXPLAIN ANALYZE and the weird thing is that the first DELETE ran pretty quickly (within seconds), while the second DELETE took so long (over 20 min) I eventually cancelled it.

Adding an index to the temp table helped the performance quite a bit.

Here's a query plan of the first DELETE for anyone interested:

 Hash Join  (cost=184.64..7854.69 rows=256482 width=6) (actual time=54.089..660.788 rows=27295 loops=1)     Hash Cond: ("outer".id = "inner".employee_id)     ->  Seq Scan on Employees  (cost=0.00..3822.82 rows=256482 width=10) (actual time=15.218..351.978 rows=256482 loops=1)     ->  Hash  (cost=184.14..184.14 rows=200 width=4) (actual time=38.807..38.807 rows=10731 loops=1)           ->  HashAggregate  (cost=182.14..184.14 rows=200 width=4) (actual time=19.801..28.773 rows=10731 loops=1)                 ->  Seq Scan on EmployeesToDelete  (cost=0.00..155.31 rows=10731 width=4) (actual time=0.005..9.062 rows=10731 loops=1)     Total runtime: 935.316 ms  (7 rows)  

At this point, I'll stick with the first DELETE unless I can find a better way of writing it.


Solution:1

Don't guess, measure. Try the various methods and see which one is the shortest to execute. Also, use EXPLAIN to know what PostgreSQL will do and see where you can optimize. Very few PostgreSQL users are able to guess correctly the fastest query...


Solution:2

I'm wondering if the following works and is more efficient?

    DELETE      FROM    Employees e      USING   EmployeesToDelete ed      WHERE   id = ed.employee_id;  

This totally depend on your index selectivity.

PostgreSQL tends to employ MERGE IN JOIN for IN predicates, which has stable execution time.

It's not affected by how many rows satisfy this condition, provided that you already have an ordered resultset.

An ordered resultset requires either a sort operation or an index. Full index traversal is very inefficient in PostgreSQL compared to SEQ SCAN.

The JOIN predicate, on the other hand, may benefit from using NESTED LOOPS if your index is very selective, and from using HASH JOIN is it's inselective.

PostgreSQL should select the right one by estimating the row count.

Since you have 30k rows against 260K rows, I expect HASH JOIN to be more efficient, and you should try to build a plan on a DELETE ... USING query.

To make sure, please post execution plan for both queries.


Solution:3

I'm not sure about the DELETE FROM ... USING syntax, but generally, a subquery should logically be the same thing as an INNER JOIN anyway. The database query optimizer should be capable (and this is just a guess) of executing the same query plan for both.


Solution:4

Why can't you delete the rows in the first place instead of adding them to the EmployeesToDelete table?

Or if you need to undo, just add a "deleted" flag to Employees, so you can reverse the deletion, or make in permanent, all in one table?


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