Date: Wed, 11 Jul 2007 22:10:49 +0200
From: Adriaan van Os <postgres@microbizz.nl>
Subject: TRUNCATE TABLE
To: pgsql-performance@postgresql.org
Message-id: <46953949.3030201@microbizz.nl>
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Recently, I have been doing extensive profiling of a version 8.1.4 Postgres DB with about 175 
tables and 5 GB of data (the server running on Fedora Linux and the clients on Windows XP). 
Surprisingly, one of the bottlenecks is TRUNCATE TABLE and that command is really slow as compared 
to other operations. For example, we have operations like:

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

where do_something is using the data in my_temporary_table to do something like a JOIN or a mass 
UPDATE or whatever.

Now, it turns out that typically most time is lost in TRUNCATE TABLE, in fact it spoils the 
performance of most operations on the DB !

I read in a mailing list archive that TRUNCATE TABLE is slow since it was made transaction-safe 
somewhere in version 7, but for operations on a temporary table (with data coming from the outside 
world) that is irrelevant, at least for my application, in casu, a middleware software package.

So, my questions are

1. Why is TRUNCATE TABLE so slow (even if transaction-safe)
2. Is there is way to dig up in the source code somewhere a quick-and-dirty TRUNCATE TABLE 
alternative for operations on temporary tables that need not be transaction-safe (because the 
middleware itself can easily restore anything that goes wrong there).

I noticed, by the way, that removing records in general is painfully slow, but I didn't do a 
detailed analysis of that issue yet.

As an alternative to TRUNCATE TABLE I tried to CREATE and DROP a table, but that wasn't any faster.

Sincerely,

Adriaan van Os