Re: still gin index creation takes forever

> Yeah, I'm not convinced either.  Still, Teodor's theory should be easily
> testable: set synchronize_seqscans to FALSE and see if the problem goes
> away.

Test suit to reproduce the problem:
    DROP TABLE IF EXISTS foo;
    DROP TABLE IF EXISTS footmp;

    CREATE OR REPLACE FUNCTION gen_array()
    RETURNS _int4 AS
    $$
        SELECT ARRAY(
            SELECT (random()*1000)::int
            FROM generate_series(1,10+(random()*90)::int)
        )
    $$
    LANGUAGE SQL VOLATILE;

    SELECT gen_array() AS v INTO foo FROM generate_series(1,100000);

    VACUUM ANALYZE foo;

    CREATE INDEX fooidx ON foo USING gin (v);
    DROP INDEX fooidx;

    SELECT * INTO footmp FROM foo LIMIT 90000;

    CREATE INDEX fooidx ON foo USING gin (v);
    DROP INDEX fooidx;

On my notebook with HEAD and default postgresql.conf it produce (show only
interesting part):

    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    Time: 14961,409 ms
    postgres=# SELECT * INTO footmp FROM foo LIMIT 90000;
    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    LOG:  checkpoints are occurring too frequently (12 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    LOG:  checkpoints are occurring too frequently (8 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    LOG:  checkpoints are occurring too frequently (7 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    LOG:  checkpoints are occurring too frequently (10 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    LOG:  checkpoints are occurring too frequently (8 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    CREATE INDEX
    Time: 56286,507 ms

So, time for creation is 4-time bigger after select.
Without "SELECT * INTO footmp FROM foo LIMIT 90000;":
    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    CREATE INDEX
    Time: 13894,050 ms
    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    LOG:  checkpoints are occurring too frequently (14 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    CREATE INDEX
    Time: 15087,348 ms

Near to the same time.

With   synchronize_seqscans = off and SELECT:
    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    CREATE INDEX
    Time: 14452,024 ms
    postgres=# SELECT * INTO footmp FROM foo LIMIT 90000;
    postgres=# CREATE INDEX fooidx ON foo USING gin (v);
    LOG:  checkpoints are occurring too frequently (16 seconds apart)
    HINT:  Consider increasing the configuration parameter "checkpoint_segments".
    CREATE INDEX
    Time: 14557,750 ms

Again, near to the same time.


--
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/