Index AM's can cache stuff in RelationData->rd_amcache. In the zedstore
table AM we've been hacking on, I'd like to also use rd_amcache to cache
some information, but that's not currently possible, because rd_amcache
can only be used by index AMs, not table AMs.
Attached patch allows rd_amcache to also be used by table AMs.
While working on this, I noticed that the memory management of relcache
entries is quite complicated. Most stuff that's part of a relcache entry
is allocated in CacheMemoryContext. But some fields have a dedicated
memory context to hold them, like rd_rulescxt for rules and rd_pdcxt for
partition information. And indexes have rd_indexcxt to hold all kinds of
index support info.
In the patch, I documented that rd_amcache must be allocated in
CacheMemoryContext, or in rd_indexcxt if it's an index. It works, but
it's a bit weird. It would nice to have one memory context in every
relcache entry, to hold all the stuff related to it, including
rd_amcache. In other words, it would be nice if we had "rd_indexcxt" for
tables, too, not just indexes. That would allow tracking memory usage
more accurately, if you're debugging an out of memory situation for example.
However, the special contexts like rd_rulescxt and rd_pdcxt would still
be needed, because of the way RelationClearRelation preserves them, when
rebuilding the relcache entry for an open relation. So I'm not sure how
much it would really simplify things. Also, there's some overhead for
having extra memory contexts, and some people already complain that the
relcache uses too much memory.
Alternatively, we could document that rd_amcache should always be
allocated in CacheMemoryContext, even for indexes. That would make the
rule for pg_amcache straightforward. There's no particular reason why
rd_amcache has to be allocated in rd_indexcxt, except for how it's
accounted for in memory context dumps.
- Heikki
>From 0b061f3c0ec8f8dedaf30b4e3cefc0cb24630ced Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakan...@iki.fi>
Date: Fri, 14 Jun 2019 18:09:19 +0300
Subject: [PATCH 1/1] Allow table AM's to use rd_amcache, too.
The rd_amcache allows an index AM to cache arbitrary information in
a relcache entry. This commit moves the cleanup of rd_amcache so that
it's also available for table AMs.
---
src/backend/utils/cache/relcache.c | 11 +++++++++--
src/include/utils/rel.h | 19 +++++++++++--------
2 files changed, 20 insertions(+), 10 deletions(-)
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 2b992d78327..204e1eb7aad 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -2358,6 +2358,10 @@ RelationDestroyRelation(Relation relation, bool remember_tupdesc)
pfree(relation->rd_options);
if (relation->rd_indextuple)
pfree(relation->rd_indextuple);
+ if (relation->rd_amcache)
+ pfree(relation->rd_amcache);
+ if (relation->rd_fdwroutine)
+ pfree(relation->rd_fdwroutine);
if (relation->rd_indexcxt)
MemoryContextDelete(relation->rd_indexcxt);
if (relation->rd_rulescxt)
@@ -2370,8 +2374,6 @@ RelationDestroyRelation(Relation relation, bool remember_tupdesc)
MemoryContextDelete(relation->rd_pdcxt);
if (relation->rd_partcheckcxt)
MemoryContextDelete(relation->rd_partcheckcxt);
- if (relation->rd_fdwroutine)
- pfree(relation->rd_fdwroutine);
pfree(relation);
}
@@ -2416,6 +2418,11 @@ RelationClearRelation(Relation relation, bool rebuild)
*/
RelationCloseSmgr(relation);
+ /* Free AM cached data, if any */
+ if (relation->rd_amcache)
+ pfree(relation->rd_amcache);
+ relation->rd_amcache = NULL;
+
/*
* Treat nailed-in system relations separately, they always need to be
* accessible, so we can't blow them away.
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index d7f33abce3f..9080a3c1609 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -152,13 +152,6 @@ typedef struct RelationData
* those with lefttype and righttype equal to the opclass's opcintype. The
* arrays are indexed by support function number, which is a sufficient
* identifier given that restriction.
- *
- * Note: rd_amcache is available for index AMs to cache private data about
- * an index. This must be just a cache since it may get reset at any time
- * (in particular, it will get reset by a relcache inval message for the
- * index). If used, it must point to a single memory chunk palloc'd in
- * rd_indexcxt. A relcache reset will include freeing that chunk and
- * setting rd_amcache = NULL.
*/
MemoryContext rd_indexcxt; /* private memory cxt for this stuff */
/* use "struct" here to avoid needing to include amapi.h: */
@@ -173,9 +166,19 @@ typedef struct RelationData
Oid *rd_exclops; /* OIDs of exclusion operators, if any */
Oid *rd_exclprocs; /* OIDs of exclusion ops' procs, if any */
uint16 *rd_exclstrats; /* exclusion ops' strategy numbers, if any */
- void *rd_amcache; /* available for use by index AM */
Oid *rd_indcollation; /* OIDs of index collations */
+ /*
+ * rd_amcache is available for index and table AMs to cache private data
+ * about the relation. This must be just a cache since it may get reset
+ * at any time (in particular, it will get reset by a relcache inval
+ * message for the relation). If used, it must point to a single memory
+ * chunk palloc'd in CacheMemoryContext, or in rd_indexcxt for an index
+ * relation. A relcache reset will include freeing that chunk and setting
+ * rd_amcache = NULL.
+ */
+ void *rd_amcache; /* available for use by index/table AM */
+
/*
* foreign-table support
*
--
2.20.1
