Re: Indices for select count(*)?

Greg Stark wrote:
> Alexander Scholz <alexander.scholz1@freenet.de> writes:
>
>>Hi, thank you for your answer.
>>
>>Regarding the performance flow when trying to find out how many records are
>>currently being stored in the table, I don't see how an index should help...
>>Nevertheless we've created an unique index on "ID" but SELECT count("ID") from
>>"XYZ" still takes 35 seconds*. (ID is the primary key basing on a sequence,
>>select count(*) isn't faster.)
>>
>>So - what kind of indexing would speed this up then?
>
>
> No form of indexing can speed this up. To answer the server has to look at
> every record and count up how many of them should be included in your result.

Why couldn't it be possible to count # of items in an index?
The density of the information (items/inode|block|whatever it's called
in btrees) is likely to be much higher giving less disk i/o.

I'm sorry if this has been discussed recently.

Best regards,
Marcus