From: Bruce Momjian <bruce@momjian.us>
Message-Id: <201006241503.o5OF3go14004@momjian.us>
Subject: Re: ALTER Table and CLUSTER does adding a new
	column rewrite clustered? (8.4.3)
In-Reply-To: <37F2FA33-49B6-478D-BCFE-FD0DBC4CBBA8@richrelevance.com>
To: Scott Carey <scott@richrelevance.com>
Date: Thu, 24 Jun 2010 11:03:42 -0400 (EDT)
CC: postgres performance list <pgsql-performance@postgresql.org>
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Scott Carey wrote:
> v. 8.4.3 
> 
> I have a table that has several indexes, one of which the table is
> clustered on.  If I do an ALTER TABLE Foo ADD COLUMN bar integer not
> null default -1;
> 
> It re-writes the whole table.

All good questions:

> * Does it adhere to the CLUSTER property of the table and write the new
> version clustered?

The new table is the exact same heap ordering as the old table;  it does
not refresh the clustering if the table has become unclustered.

> * Does it properly write it with the FILLFACTOR setting?

Yes, inserts are used to populate the new table, and inserts honor
FILLFACTOR.

> * Are all the indexes re-created too, or are they bloated and need a REINDEX?

They are recreated.

> http://www.postgresql.org/docs/8.4/static/sql-altertable.html 
>   does not seem to answer the above, it mentions the conditions that
> cause a rewrite but does not say what the state is after the rewrite
> with respect to CLUSTER, FILLFACTOR, and index bloat.

I have added a documentation patch to mention the indexes are rebuilt; 
applied patch attached.

The gory details can be found in src/backend/commands/tablecmds.c.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + None of us is going to be here forever. +

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Index: doc/src/sgml/ref/alter_table.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/ref/alter_table.sgml,v
retrieving revision 1.114
diff -c -c -r1.114 alter_table.sgml
*** doc/src/sgml/ref/alter_table.sgml	9 Jun 2010 17:48:10 -0000	1.114
--- doc/src/sgml/ref/alter_table.sgml	24 Jun 2010 14:54:00 -0000
***************
*** 689,696 ****
  
     <para>
      Adding a column with a non-null default or changing the type of an
!     existing column will require the entire table to be rewritten.  This
!     might take a significant amount of time for a large table; and it will
      temporarily require double the disk space.  Adding or removing a system
      <literal>oid</> column likewise requires rewriting the entire table.
     </para>
--- 689,696 ----
  
     <para>
      Adding a column with a non-null default or changing the type of an
!     existing column will require the entire table and indexes to be rewritten.
!     This might take a significant amount of time for a large table; and it will
      temporarily require double the disk space.  Adding or removing a system
      <literal>oid</> column likewise requires rewriting the entire table.
     </para>

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