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Re: HOT - whats next ?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Pavan Deolasee" <pavan(dot)deolasee(at)enterprisedb(dot)com>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: HOT - whats next ?
Date: 2007-03-02 15:08:19
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Lists: pgsql-hackers
"Pavan Deolasee" <pavan(dot)deolasee(at)enterprisedb(dot)com> writes:
> - Another problem with the current HOT patch is that it generates
>   tuple level fragmentation while reusing LP_DELETEd items when
>   the new tuple is of smaller size than the original one. Heikki
>   supported using best-fit strategy to reduce the fragmentation
>   and thats worth trying. But ISTM that we can also correct
>   row-level defragmentation whenever we run out of free space
>   and LP_DELETEd tuples while doing UPDATE. Since this does not
>   require moving tuples around, we can do this by a simple EXCLUSIVE
>   lock on the page.

You are mistaken.  To move existing tuples requires
LockBufferForCleanup, the same as VACUUM needs; otherwise some other
backend might continue to access a tuple it found previously.

How much testing of this patch's concurrent behavior has been done?
I'm wondering if any other locking thinkos are in there ...

			regards, tom lane

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