Re: single index on more than two coulumns a bad thing?

another thing that I have all over the place is a hierarchy:
index on grandfather_table(grandfather)
index on father_table(grandfather, father)
index on son_table(grandfather, father, son)

almost all of my indices are composite. Are you thinking about composite
indices with low cardinality leading columns?

/Aaron

----- Original Message -----
From: "Josh Berkus" <josh@agliodbs.com>
To: "Palle Girgensohn" <girgen@pingpong.net>;
<pgsql-performance@postgresql.org>
Sent: Thursday, April 01, 2004 7:35 PM
Subject: Re: [PERFORM] single index on more than two coulumns a bad thing?


> Palle,
>
> > Is it always bad to create index xx on yy (field1, field2, field3);
>
> No, it seldom bad, in fact.    I have some indexes that run up to seven
> columns, becuase they are required for unique keys.
>
> Indexes of 3-4 columns are often *required* for many-to-many join tables.
>
> I'm afraid that you've been given some misleading advice.
>
> > I guess the problem is that the index might often grow bigger than the
> > table, or at least big enough not to speed up the queries?
>
> Well, yes ... a 4-column index on a 5-column table could be bigger than
the
> table if allowed to bloat and not re-indexed.   But that's just a reason
for
> better maintainence.
>
> --
> -Josh Berkus
>  Aglio Database Solutions
>  San Francisco
>
>
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