On 10/27/2013 02:23 PM, Robert James wrote:
On 10/27/13, Thomas Kellerer <spam_eater@xxxxxxx> 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?
Is there a FK relationship involved?
Could we see the schema for another_table?
Or should I be using a different type of table for work tables? (RAM only table)
--
Adrian Klaver
adrian.klaver@xxxxxxxxx
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