Re: Index Choice Problem

Unfortunately I'm using 8.0.4 and this is for a government website, I only get so many maintenance windows. Is this the only workaround for this issue?

I did make a test index as you described on my test box and tried the query and it used the new index. However, ORDER BY forum_id then last_post_time is simply not the intended sorting order. (Though I'm considering just SELECTing the topic_last_post_time field and resorting the results in the script if this is the only workaround).

- Adam

On 2/18/06, Tom Lane < tgl@sss.pgh.pa.us> wrote:

Adam Alkins <adam.alkins@gmail.com> writes:
> SELECT t.topic_id
>                       FROM phpbb_topics AS t
>                               WHERE t.forum_id = 71
>                                       AND t.topic_id NOT IN (205026, 29046, 144569, 59780, 187424,
> 138635, 184973, 170551, 22419, 181690, 197254, 205130)
>                                               ORDER BY t.topic_last_post_time DESC
>                                                       LIMIT 23 OFFSET 0

If you're using 8.1, you'd probably find that an index on (forum_id,
topic_last_post_time) would work nicely for this.  You could use it
in prior versions too, but you'd have to spell the ORDER BY rather
strangely:
        ORDER BY forum_id desc, topic_last_post_time desc
The reason for this trickery is to get the planner to realize that
the index order matches the ORDER BY ...

                        regards, tom lane

--

Adam Alkins
http://www.rasadam.com
Mobile: 868-680-4612