autovacuum, reloptions, and hardcoded pg_class tupdesc
Hi,
So I've been progressing on revising the autovacuum patch to make it
work with the current reloptions. We have a number of options:
1. Call heap_open() for every relation that we're going to check, and
examine the reloptions via the relcache.
I'm not sure that I like this very much. Right now we just plow
ahead using a pg_class seqscan, which avoids locking the relations
just for the sake of verifying whether they need work. However, this
is the cleanest option in terms of modularity breakage.
2. Play some dirty tricks in autovacuum and extract the reloptions from
the bare pg_class tuple ourselves. I think doing this cleanly
requires exporting some internal functions like
GetPgClassDescriptor() from relcache.c.
3. Disallow setting reloptions for pg_class, which (I think) allows us
to avoid having to use GetPgClassDescriptor, but the rest of it is
still a dirty hack.
In particular, if we do either (2) or (3), we'll need to keep an eye on
that code whenever we modify RelationParseRelOptions.
Thoughts?
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.
Alvaro Herrera <alvherre@commandprompt.com> writes:
So I've been progressing on revising the autovacuum patch to make it
work with the current reloptions. We have a number of options:
1. Call heap_open() for every relation that we're going to check, and
examine the reloptions via the relcache.
I'm not sure that I like this very much.
I don't like it at all, mainly because it implies taking locks on tables
that autovacuum doesn't need to be touching. Even if it's only
AccessShareLock, it could result in problems vis-a-vis clients that are
holding exclusive table locks.
Right now we just plow
ahead using a pg_class seqscan, which avoids locking the relations
just for the sake of verifying whether they need work.
We should stick with that, and refactor the reloptions code as needed to
be able to work from just a pg_class tuple. I'm envisioning a scheme
like:
bottom level: extract from pg_class tuple, return a palloc'd struct
relcache: logic to cache the result of the above
top level: exported function to return a cached options struct
The autovac scan could use the bottom-level API.
regards, tom lane
Tom Lane wrote:
Alvaro Herrera <alvherre@commandprompt.com> writes:
Right now we just plow
ahead using a pg_class seqscan, which avoids locking the relations
just for the sake of verifying whether they need work.We should stick with that, and refactor the reloptions code as needed to
be able to work from just a pg_class tuple. I'm envisioning a scheme
like:bottom level: extract from pg_class tuple, return a palloc'd struct
relcache: logic to cache the result of the above
top level: exported function to return a cached options struct
The autovac scan could use the bottom-level API.
I'm not sure that we have any use for the top level you propose; the
attached patch just uses the two lower levels, and I think it fits
autovacuum usage just fine. Thanks for the idea.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
Attachments:
relcache-reloptions.patchtext/x-diff; charset=us-asciiDownload+76-32
Alvaro Herrera wrote:
I'm not sure that we have any use for the top level you propose; the
attached patch just uses the two lower levels, and I think it fits
autovacuum usage just fine. Thanks for the idea.
Of course, there's no need to pass the relkind; it goes inside the
pg_class tuple.
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.
Attachments:
relcache-reloptions-1.1.patchtext/x-diff; charset=us-asciiDownload+7-5
Alvaro Herrera wrote:
Alvaro Herrera wrote:
I'm not sure that we have any use for the top level you propose; the
attached patch just uses the two lower levels, and I think it fits
autovacuum usage just fine. Thanks for the idea.Of course, there's no need to pass the relkind; it goes inside the
pg_class tuple.
Applied.
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.