| From: | Torsten Förtsch <torsten(dot)foertsch(at)gmx(dot)net> |
|---|---|
| To: | Thomas Kellerer <spam_eater(at)gmx(dot)net>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Disable an index temporarily |
| Date: | 2014-04-20 11:19:09 |
| Message-ID: | 5353AD2D.7070101@gmx.net |
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| Lists: | pgsql-general |
On 20/04/14 12:08, Thomas Kellerer wrote:
>> Example:
>>
>> btree (fmb_id, action_type)
>> btree (fmb_id)
>>
[...]
>
> Btw: in the above example the second one is definitely not needed.
> Any query that uses the second one (single column) can also use the
> first one.
I know.
But the single column index is 3534 MB the 2-column one 4963 MB. The
number of rows per distinct fmb_id is very small, usually 1 or 2. So, if
a query looks for "fmb_id=A and action_type=B", it has to filter out
only one row if the first index is not available. Hence, I thought maybe
the lower size of the index and the higher compactness per page could
outweigh the more direct access provided by the 2-column index.
I am quite sure there is no query that qualifies for an index-only scan
on the 2-column index.
Torsten
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