comment on

I don't know what's wrong with it -- you should try some place specialising in MySQL. It seems unlikely that Perl/DBI are at fault unless RHEL has done something wonky with the relevant packages.

Anyway, you seem to be doing a death by a thousand queries there and that means _many_ round-trips. I'm sure the script can be rewritten to perform a single query. This is how I'd do it with Postgres-specific syntax:

SELECT DISTINCT ON (id) id, description
FROM list_index li
ORDER BY id, rpt_key DESC;
[download]

Writing it with a correlated subquery, we get a query that is terribly slow (one second on my couple-of-thousand-rows dataset):

SELECT DISTINCT id,
    (SELECT description from list_index li_i
     WHERE li_i.id = li_o.id
     ORDER BY created_at DESC limit 1) AS description
    FROM list_index li_o;
[download]

But moving the distinctness to a subquery makes it fast (~7 milliseconds):

SELECT id,
    (SELECT description from list_index li_i
     WHERE li_i.id = li_o.id
     ORDER BY created_at DESC limit 1) AS description
    FROM
        (SELECT DISTINCT id FROM list_index) li_o;
[download]

The SQL should work on every dialect, but of course, these speed measurements apply only to the PostgreSQL query planner. MySQL's is different. Try and see.

...My brains aren't working well enough right now to produce a way to do it without a dependent subquery.

In reply to Re: Much slower DBI on RHEL6 by Anonymous Monk
in thread Much slower DBI on RHEL6 by MPM

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