Make large sequential scans and VACUUMs work in a limited-size "ring" of

buffers, rather than blowing out the whole shared-buffer arena.  Aside from
avoiding cache spoliation, this fixes the problem that VACUUM formerly tended
to cause a WAL flush for every page it modified, because we had it hacked to
use only a single buffer.  Those flushes will now occur only once per
ring-ful.  The exact ring size, and the threshold for seqscans to switch into
the ring usage pattern, remain under debate; but the infrastructure seems
done.  The key bit of infrastructure is a new optional BufferAccessStrategy
object that can be passed to ReadBuffer operations; this replaces the former
StrategyHintVacuum API.

This patch also changes the buffer usage-count methodology a bit: we now
advance usage_count when first pinning a buffer, rather than when last
unpinning it.  To preserve the behavior that a buffer's lifetime starts to
decrease when it's released, the clock sweep code is modified to not decrement
usage_count of pinned buffers.

Work not done in this commit: teach GiST and GIN indexes to use the vacuum
BufferAccessStrategy for vacuum-driven fetches.

Original patch by Simon, reworked by Heikki and again by Tom.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 2754a6db6a..d77aec2dd7 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.107 2007/04/30 03:23:48 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.108 2007/05/30 20:11:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -63,10 +63,13 @@ typedef struct AnlIndexData
 /* Default statistics target (GUC parameter) */
 int			default_statistics_target = 10;
 
+/* A few variables that don't seem worth passing around as parameters */
 static int	elevel = -1;
 
 static MemoryContext anl_context = NULL;
 
+static BufferAccessStrategy vac_strategy;
+
 
 static void BlockSampler_Init(BlockSampler bs, BlockNumber nblocks,
 				  int samplesize);
@@ -94,7 +97,8 @@ static bool std_typanalyze(VacAttrStats *stats);
  *	analyze_rel() -- analyze one relation
  */
 void
-analyze_rel(Oid relid, VacuumStmt *vacstmt)
+analyze_rel(Oid relid, VacuumStmt *vacstmt,
+			BufferAccessStrategy bstrategy)
 {
 	Relation	onerel;
 	int			attr_cnt,
@@ -120,6 +124,8 @@ analyze_rel(Oid relid, VacuumStmt *vacstmt)
 	else
 		elevel = DEBUG2;
 
+	vac_strategy = bstrategy;
+
 	/*
 	 * Use the current context for storing analysis info.  vacuum.c ensures
 	 * that this context will be cleared when I return, thus releasing the
@@ -845,7 +851,7 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 		 * looking at it.  We don't maintain a lock on the page, so tuples
 		 * could get added to it, but we ignore such tuples.
 		 */
-		targbuffer = ReadBuffer(onerel, targblock);
+		targbuffer = ReadBufferWithStrategy(onerel, targblock, vac_strategy);
 		LockBuffer(targbuffer, BUFFER_LOCK_SHARE);
 		targpage = BufferGetPage(targbuffer);
 		maxoffset = PageGetMaxOffsetNumber(targpage);