Re: New VACUUM FULL

On Tue, 2009-10-27 at 13:55 +0900, Itagaki Takahiro wrote:
> Here is a patch to support "rewrite" version of VACUUM FULL.
> It was called "VACUUM REWRITE" in the past disucussin,
> but I choose the following syntax for now:
> 
>     VACUUM ( FULL [ INPLACE | REPLACE ] )  [ table_name ]
> 
> The reason is to contrast the new REPLACE behavior with the old INPLACE
> behavior. I cannot find any good terms of opposite meaning of REWRITE.
> 
> The new version works like as CLUSTER USING ctid or rewriting in
> ALTER TABLE. It must be faster than them if we have many dead tuples
> and are not interested in physical order of tuples.
> 
> We still need traditional VACUUM FULL behavior for system catalog because
> we cannot change relfilenode for them. Also, VACUUM FULL REPLACE is not
> always better than traditional VACUUM FULL; the new version requires
> additional disk space and might be slower if we have a few dead tuples.
> 
> "VACUUM FULL" and "VACUUM (FULL)" are synonyms for "VACUUM (FULL REPLACE)",
> but if the target table is s system catalog, instead "FULL INPLACE" is
> used automatically.

> If this approach is reasonable, I'll go for other supplemental parts.
> (documentations, vacuumdb, etc.)

Rough approach looks fine to me.

The internal logic is fairly hard to read. I'd suggest you have option
flags VACUUM_FULL and VACUUM_REPLACE, rather than VACUUM_INPLACE and
VACUUM_REPLACE. So VACUUM_REPLACE can only be set iff VACUUM_FULL. That
will reduce much of the logic.

-- Simon Riggs           www.2ndQuadrant.com