Re: index use again and again

On Tue, 12 Feb 2002, Tom Lane wrote:

> Holger Marzen <holger@marzen.de> writes:
> > Are 152428 rows not enough to use the index?
>
> More like "there are too many rows to use the index".
>
> You've got a query that is estimated to hit 22322/152428 = 15% of the
> table.  If the rows in question are uniformly scattered through the
> table then the system will certainly have to read every block of the
> table to get them all.  It may as well read the table sequentially,
> rather than do the extra I/O to read the index too.

You're right. When the result is small, the index is used. When the
number of rows is a little bit higher and postgres uses a table scan,
the query is much slower. The table is not very unordered. Can I force
postgres to raise the limit where it starts scanning the whole table?

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