Re: [GENERAL] Yet Another newbie not understanding why an index isn't used

Are you using btree indexes? It of course won't work for hash.

Felix Morley Finch wrote:
>
> I have a simple table of data from temperature probes, with two indices:
>
> hlt=> \d hltdata
> Table = hltdata
> +------------------+------------------+-------+
> | Field | Type | Length|
> +------------------+------------------+-------+
> | id | int2 | 2 |
> | stmp | datetime | 8 |
> | raw | int2 | 2 |
> | cooked | int2 | 2 |
> +------------------+------------------+-------+
>
> hlt=> \d hltdata_all
> +------------------+------------------+-------+
> | id | int2 | 2 |
> | stmp | datetime | 8 |
> +------------------+------------------+-------+
> (hltdata_all is a UNIQUE index to prevent dups.)
>
> hlt=> \d hltdata_stmp
> +------------------+------------------+-------+
> | stmp | datetime | 8 |
> +------------------+------------------+-------+
>
> There are 1.5 million rows. It has been VACUUMed since the indices
> were initially created in the COPY, and no rows have been added
> since. It would seem like a perfect opportunity for the following
> SELECT to use the indices; just grab the first 10 rows. Yet it grinds
> for 93 seconds on a PII 450 w/ 256M of RAM. It's only 77 seconds if I
> order by stmp alone.
>
> hlt=> EXPLAIN SELECT * FROM hltdata ORDER BY stmp, id LIMIT 10;
> NOTICE: QUERY PLAN:
>
> Sort (cost=62394.19 rows=1584824 width=14)
> -> Seq Scan on hltdata (cost=62394.19 rows=1584824 width=14)
>
> The only entry in the FAQ prompted me to do the VACUUM, but I didn't
> see anything else related to this. I reckond it must be pretty basic,
> though...
>
> --
> ... _._. ._ ._. . _._. ._. ___ .__ ._. . .__. ._ .. ._.
> Felix Finch: scarecrow repairman & rocket surgeon / felix(at)crowfix(dot)com
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> I've found a solution to Fermat's Last Theorem but I see I've run out of room o