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Re: SSI modularity questions

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: SSI modularity questions
Date: 2011-06-28 18:29:35
Message-ID: 4E0A1D8F.7060909@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 28.06.2011 20:47, Kevin Grittner wrote:
> (3)  Heap tuples are locked so that updates or deletes by an
> overlapping transaction of the tuple which has been read can be
> detected as a rw-conflict.  Keep in mind that access for such a
> delete or update may not go through the same index on which the
> conflicting read occurred.  It might use a different index or a
> seqscan.  These may be promoted to page or heap relation locks to
> control the shared space used by predicate locks, but the concept is
> the same -- we're locking actual tuples read, not any gaps.

Ok, that's what I was missing. So the predicate locks on heap tuples are 
necessary. Thanks for explaining this again.

> So, the heap relation lock is clearly needed for the seqscan.  There
> is room for performance improvement there in skipping the tuple lock
> attempt when we're in a seqscan, which will always be a no-op when it
> finds the heap relation lock after a hash table lookup.  But you are
> also questioning whether the predicate locking of the tuples where
> rs_relpredicatelocked is tested can be removed entirely, rather than
> conditioned on the boolean.  The question is: can the code be reached
> on something other than a seqscan of the heap, and can this happen
> for a non-temporary, non-system table using a MVCC snapshot?
>
> I've been trying to work backward to all the spots which call these
> functions, directly or indirectly to determine that.  That's
> obviously not trivial or easy work, and I fear that unless someone
> more familiar with the code than I can weigh in on that question for
> any particular PredicateLockTuple() call, I would rather leave the
> calls alone for 9.1 and sort this out in 9.2.  I'm confident that
> they don't do any damage where they are; it's a matter of very
> marginal performance benefit (skipping a call to a fast return) and
> code tidiness (not making unnecessary calls).

Hmm, the calls in question are the ones in heapgettup() and 
heapgettup_pagemode(), which are subroutines of heap_getnext(). 
heap_getnext() is only used in sequential scans, so it seems safe to 
remove those calls.

>>> (2) In reviewing the above, Heikki noticed that there was a second
>>> place in the executor that SSI calls were needed but missing. I
>>> submitted a patch here:
>>>
>>>
> http://archives.postgresql.org/message-id/4E07550F020000250003EC42@gw.wicourts.gov
>>>
>>> I wonder, though, whether the section of code which I needed to
>>> modify should be moved to a new function in heapam.c on modularity
>>> grounds.
>>>
>>> If these two places were moved, there would be no SSI calls from
>>> any source file in the executor subdirectory.
>>
>> Same here, we might not need those PredicateLockTuple calls in
>> bitmap heap scan at all. Can you check my logic, and verify if
>> those PredicateLockTuple() calls are needed?
>
> These sure look like they are needed per point (3) above.

Yep.

> I would
> like to add a test involving a lossy bitmap scan.  How many rows are
> normally needed to force a bitmap scan to be lossy?

The size of bitmaps is controlled by work_mem, so you can set work_mem 
very small to cause them to become lossy earlier. Off the top of my head 
I don't have any guesstimate on how many rows you need.

>  What's the
> easiest way to check whether a plan is going to use (or is using) a
> lossy bitmap scan?

Good question. There doesn't seem to be anything in the EXPLAIN ANALYZE 
output to show that, so I think you'll have to resort to adding some 
elog()s in the right places.

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

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