Re: index not used in joins

Sebastian Böck wrote:
> Richard Huxton wrote:
>
>> Sebastian Böck wrote:
>>
>>> Richard Huxton wrote:
>>>
>>>> Can you post the output from your "explain analyse" calls too? The
>>>> statistics aren't going to be the same on different machines.
>>>>
>>>
>>> Sure, here it is.
>>
>>
>>
>> Thanks. (PS - remember to cc the list too).
>
>
> [output of EXPLAIN ANALYZE]
>
>> OK - so what you want to know is why index "test_999" is used in the
>> second but not the first, even though both return the same rows.
>>
>> The fact is that the conditional index:
>> CREATE INDEX test_999 ON test (datum)
>> WHERE version = '999' OR approved IS NOT NULL;
>> AFAIK looks at the WHERE clause of your query to determine where it
>> can run. Don't forget that the planner needs to pick which index is
>> best *before* it starts fetching data.
>>
>> So - in the first example there might be rows where e.g. t.version=998
>> which means test_999 would be a poor choice of index.
>
>
> But what if the table users contains only 1 row and the column "version"
> has a value of "999"?

It still doesn't know that the only value in "version" is 999(*). Let's
say there were 2000 rows and 1900 had the value 999 - the index is still
useless because we'd have to do a sequential scan to check the remaining
200 rows.

> Are there any other options to speed up this kind of query?

Well, your problem is the (version=X OR approved IS NOT NULL) clause. I
must admit I can't quite see what this is supposed to do. The "test"
table connects to the "users" table via "version" (and "datum", though
not a simple check) unless the "test" has been "approved", in which case
it applies to all users?
Can you explain what the various tables/columns are really for?

(*) Don't forget the statistics for column values are usually
out-of-date compared to the actual data, so you can't rely on it.

--
Richard Huxton
Archonet Ltd