Re: [Devel 9.2] Index-only scan in count aggregation

On Wed, Apr 18, 2012 at 12:52 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Apr 18, 2012 at 12:40 PM, Emanuel Calvo <postgres(dot)arg(at)gmail(dot)com> wrote:
>>> I'm not sure what you're unhappy about.  It seems that the query
>>> planner picked the fastest plan (a sequential scan) and then when you
>>> disabled that it picked the second-fastest plan (an index-only scan).
>>>
>>> The index-only scan would have a chance of beating the sequential scan
>>> if the table had been recently vacuumed, but not in the case where
>>> every row is going to require a heap fetch.
>>
>> Oh, I see now. Honestly, I thought it wasn't necessary to make a heap
>> fetch. The table
>> doesn't have any modifications, but with the vacuum the cost changed.
>
> Ah, I see.  Yeah, I think you're not going to be the first person to
> not realize that, especially since we haven't changed the rules for
> autovacuuming, and therefore you can't count on autovac to correct the
> problem for you.  :-(

And by "the first person" I of course meant "the last person".

--
Robert Haas
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