Re: Fwd: Which indexes does a query use?

Chris Velevitch wrote:
> On 2/27/06, Russell Smith <mr-russ@pws.com.au> wrote:
>
>>Not necessarily.  How many rows are there in the table at the moment.
>>If pg uses and index, it first has to get the index page, then get the
>>heap page.  So if you have a small number of blocks in the heap it's
>>actually cheaper to just scan the heap.  I would guess the heap is small
>>by the fact that the seq scan only find one row, and finds it in 46.32
>>units.  The row width isn't high either and that means you get good
>>block packing.  Probably 80-100 row's per block.
>
> It's about 200 records.
>
> Are you saying that the strategy pg uses is dynamic, in that as the
> size of the table grows the strategy changes?

Yes, it does.

For 200 rows, it's just as quick for it to look at every row. If you
have 20,000 - then that's a different case.

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