Date: Thu, 12 Jul 2007 09:37:40 +0200
From: Adriaan van Os <postgres@microbizz.nl>
Subject: Re: TRUNCATE TABLE
In-reply-to: <3561.1184190885@sss.pgh.pa.us>
To: pgsql-performance@postgresql.org
Message-id: <4695DA44.2060000@microbizz.nl>
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Gregory Stark wrote:
> That's strange. Deleting should be the *quickest* operation in Postgres. Do
> you perchance have foreign key references referencing this table?

No.

> Do you have any triggers?

No.

Tom Lane wrote:
> Adriaan van Os <postgres@microbizz.nl> writes:
>> Surprisingly, one of the bottlenecks is TRUNCATE TABLE and that
>> command is really slow as compared to other operations.
> 
> When you don't quantify that statement at all, it's hard to make an
> intelligent comment on it, but TRUNCATE per se shouldn't be slow.
> Are you sure you are not measuring a delay to obtain exclusive lock
> on the table before it can be truncated (ie, waiting for other
> transactions to finish with it)?

During the tests, there is only one connection to the database server. No other transactions are 
running.

> When you don't quantify that statement at all, it's hard to make an
> intelligent comment on it, but TRUNCATE per se shouldn't be slow.

Below are some timings, in milliseconds.

> TRUNCATE TABLE my_temporary_table
> COPY my_temporary_table ... FROM STDIN BINARY
> do_something

The temporary table has one INT4 field and no indices.

Numrows		TRUNCATE (ms)			COPY (ms)		SELECT (ms)
   5122				  80,6					16,1				51,2
   3910				  79,5					12,9				39,9
   2745				  90,4					10,7				32,4
   1568				  99,5					 7,6				24,7
    398				 161,1					 4,0				22,1
    200				  79,5					 3,3				22,0
    200				  87,9 					 3,1				22,0
222368 				4943,5				 728,6			7659,5
222368				1685,7				 512,2			2883,1

Note how fast the COPY is (which is nice). The SELECT statement uses the temporary table.

Regards,

Adriaan van Os