Simplify the effective_io_concurrency setting.

The effective_io_concurrency GUC and equivalent tablespace option were
previously passed through a formula based on a theory about RAID
spindles and probabilities, to arrive at the number of pages to prefetch
in bitmap heap scans.  Tomas Vondra, Andres Freund and others argued
that it was anachronistic and hard to justify, and commit 558a9165e08
already started down the path of bypassing it in new code.  We agreed to
drop that logic and use the value directly.

For the default setting of 1, there is no change in effect.  Higher
settings can be converted from the old meaning to the new with:

  select round(sum(OLD / n::float)) from generate_series(1, OLD) s(n);

We might want to consider renaming the GUC before the next release given
the change in meaning, but it's not clear that many users had set it
very carefully anyway.  That decision is deferred for now.

Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com

1 files changed, 3 insertions, 15 deletions

diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
index ae8a11da304..726d3a2d9a4 100644
--- a/src/backend/executor/nodeBitmapHeapscan.c
+++ b/src/backend/executor/nodeBitmapHeapscan.c
@@ -707,7 +707,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 {
 	BitmapHeapScanState *scanstate;
 	Relation	currentRelation;
-	int			io_concurrency;
 
 	/* check for unsupported flags */
 	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -737,8 +736,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	scanstate->prefetch_iterator = NULL;
 	scanstate->prefetch_pages = 0;
 	scanstate->prefetch_target = 0;
-	/* may be updated below */
-	scanstate->prefetch_maximum = target_prefetch_pages;
 	scanstate->pscan_len = 0;
 	scanstate->initialized = false;
 	scanstate->shared_tbmiterator = NULL;
@@ -794,20 +791,11 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 		ExecInitQual(node->bitmapqualorig, (PlanState *) scanstate);
 
 	/*
-	 * Determine the maximum for prefetch_target.  If the tablespace has a
-	 * specific IO concurrency set, use that to compute the corresponding
-	 * maximum value; otherwise, we already initialized to the value computed
-	 * by the GUC machinery.
+	 * Maximum number of prefetches for the tablespace if configured, otherwise
+	 * the current value of the effective_io_concurrency GUC.
 	 */
-	io_concurrency =
+	scanstate->prefetch_maximum =
 		get_tablespace_io_concurrency(currentRelation->rd_rel->reltablespace);
-	if (io_concurrency != effective_io_concurrency)
-	{
-		double		maximum;
-
-		if (ComputeIoConcurrency(io_concurrency, &maximum))
-			scanstate->prefetch_maximum = rint(maximum);
-	}
 
 	scanstate->ss.ss_currentRelation = currentRelation;