Horton

Re: heap vacuum & cleanup locks

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Jim Nasby <jim@nasby.net>
Cc: Robert Haas <robertmhaas@gmail.com>, Itagaki Takahiro <itagaki.takahiro@gmail.com>, pgsql-hackers@postgresql.org
Date: 2011-06-06T07:02:40Z
Lists: pgsql-hackers

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  1. Make VACUUM avoid waiting for a cleanup lock, where possible.

On 06.06.2011 09:35, Jim Nasby wrote:
> I've had a related idea that I haven't looked into... if you're scanning a relation (ie: index scan, seq scan) I've wondered if it would be more efficient to deal with the entire page at once, possibly be making a copy of it. This would reduce the number of times you pin the page (often quite dramatically). I realize that means copying the entire page, but I suspect that would occur entirely in the L1 cache, which would be fast.

We already do that. When an index scan moves to an index page, the heap 
tid pointers of all the matching index tuples are copied to 
backend-private memory in one go, and the lock is released. And for a 
seqscan, the visibility of all the tuples on the page is checked in one 
go while holding the lock, then the lock is released but the pin is 
kept. The pin is only released after all the tuples have been read. 
There's no repeated pin-unpin for each tuple.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com