Re: Work table - Mailing list pgsql-general

From Robert James
Subject Re: Work table
Date
Msg-id CAGYyBgguc_vx1kpwnKtom+fWV8DCKD6ih23-ugLTxJDoybQCjw@mail.gmail.com
Whole thread Raw
In response to Re: Work table  (Thomas Kellerer <spam_eater@gmx.net>)
Responses Re: Work table
Re: Work table
List pgsql-general
On 10/27/13, Thomas Kellerer <spam_eater@gmx.net> wrote:
> Robert James wrote on 27.10.2013 20:47:
>> I'm using Postgres for data analysis (interactive and batch).  I need
>> to focus the analysis on a subset of one table, and, for both
>> performance and simplicity, have a function which loads that subset
>> into another table (DELETE FROM another_table; INSERT INTO
>> another_table SELECT ...).
>>
>> Oddly enough, although the SELECT itself is very quick (< 1 s), the
>> DELETE and INSERT can take over a minute! I can't figure out why.
>> another_table is simple: it has only 7 fields.  Two of those fields
>> are indexed, using a simple one field standard index.  There are no
>> triggers on it.
>>
>> What is the cause of this behavior? What should I do to make this
>> faster? Is there a recommended work around?
>>
>> (I'm hesitant to drop another_table and recreate it each time, since
>> many views depend on it.)
>
> DELETE can be a quite lengthy thing to do - especially with a large number
> of rows.
>
> If you use TRUNCATE instead, this will be *much* quicker with the additional
> benefit,
> that if you INSERT the rows in the same transaction, the INSERT will require
> much less
> I/O because it's not logged.
>

Changing DELETE to TRUNCATE and putting it all in a transaction
brought the time down to 40 seconds.  But this is still awfully slow,
when the SELECT is under a second.

Is there another problem here? Perhaps something to do with
triggerring autovacuum?

Or should I be using a different type of table for work tables? (RAM only table)


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