Re: SSI modularity questions

There are two outstanding patches for SSI which involve questions
about modularity. In particular, they involve calls to predicate
locking and conflict detection from executor source files rather
than AM source files (where most such calls exist).

(1) Dan submitted this patch:

http://archives.postgresql.org/message-id/20110622045850.GN83336@csail.mit.edu

which is a very safe and very simple patch to improve performance on
sequential heap scans at the serializable transaction isolation
level. The location of the code being modified raised questions
about modularity. There is a reasonably clear place to which it
could be moved in the heap AM, but because it would acquire a
predicate lock during node setup, it would get a lock on the heap
even if the node was never used, which could be a performance
regression in some cases.

(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.

Should these be moved before beta3?

-Kevin

On 27.06.2011 21:23, Kevin Grittner wrote:
> There are two outstanding patches for SSI which involve questions
> about modularity. In particular, they involve calls to predicate
> locking and conflict detection from executor source files rather
> than AM source files (where most such calls exist).
>
> (1) Dan submitted this patch:
>
> http://archives.postgresql.org/message-id/20110622045850.GN83336@csail.mit.edu
>
> which is a very safe and very simple patch to improve performance on
> sequential heap scans at the serializable transaction isolation
> level. The location of the code being modified raised questions
> about modularity. There is a reasonably clear place to which it
> could be moved in the heap AM, but because it would acquire a
> predicate lock during node setup, it would get a lock on the heap
> even if the node was never used, which could be a performance
> regression in some cases.

The bigger question is if those calls are needed at all
(http://archives.postgresql.org/message-id/4E072EA9.3030200@enterprisedb.com).
I'm uneasy about changing them this late in the release cycle, but I
don't feel good about leaving useless clutter in place just because
we're late in the release cycle either. More importantly, if locking the
whole relation in a seqscan is not just a performance optimization, but
is actually required for correctness, it's important that we make the
code and comments to reflect that or someone will break it in the future.

> (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?

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