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Re: TODO-Item: B-tree fillfactor control

From: ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: TODO-Item: B-tree fillfactor control
Date: 2006-02-02 08:19:43
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Lists: pgsql-hackerspgsql-patches
Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> wrote:

> > - Is fillfactor useful for hash and gist indexes?
> >     I think hash does not need it, but gist might need it.
> Not sure.  We don't know what type of index a GIST will be so we have no
> way of knowing.  I am thinking we can implement just btree now and the
> GIST folks can add it later if they want.  My guess is that each GIST is
> going to behave differently for different fill-factors, so if allow it
> to be set for GIST, GIST developers can pull the value if they want.

My understanding about hash was wrong. It uses fill factor of 75%, which is
hard-coded. On the other hand, GIST has no ability to control fill factor
currently. I'm trying to add fill factors to hash and gist, so I'll ask
index developers to review a patch in the future.

> > - Is it appropriate to use GUC variables to control fillfactors?
> >     Is it better to extend CREATE INDEX / REINDEX grammar?
> I think it has to be part of CREATE INDEX and ALTER INDEX.

SQL standard has no regulation for indexes, so I refered to other databases.
  - Oracle and DB2 : CREATE INDEX index ON table (...) PCTFREE 30;
  - MS SQL Server  : CREATE INDEX index ON table (...) WITH FILLFACTOR = 70;

PCTFREE seems to be common, so I'll extend DDL to use PCTFREE syntax.
The following two syntaxes will be able to be used. 
  1. SET btree_free_percent = 30;
     CREATE INDEX index ON table (...);
     SET btree_free_percent = 10; -- revert
  2. CREATE INDEX index ON table (...) PCTFREE 30;

1 would be useful for a compatibe pg_dump format, per suggestion from Tom.

> Is there a use for separate node and leaf settings?

We should use different settings for leaf and node, but it may confuse users.
So I'll simplify the setting as follows:
        node_free_percent = Min(30%, 3 * leaf_free_percent)
When leaf_free_percent is 10%, node_free_percent is 30%. They are the same
values of the current implementation.

ITAGAKI Takahiro
NTT Cyber Space Laboratories

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