Moving to -hackers I was asking about how to distinguish the index cost component of an indexscan from the cost of heap. https://www.postgresql.org/message-id/20200103141427.GK12066%40telsasoft.com
On Fri, Jan 03, 2020 at 09:33:35AM -0500, Jeff Janes wrote: > > It would help to be able to set enable_bitmapscan=FORCE (to make all index > > scans go through a bitmap). > > Doesn't enable_indexscan=off accomplish this already? It is possible but > not terribly likely to switch from index to seq, rather than from index to > bitmap. (Unless the index scan was being used to obtain an ordered result, > but a hypothetical enable_bitmapscan=FORCE can't fix that). No, enable_indexscan=off implicitly disables bitmap index scans, since it does: cost_bitmap_heap_scan(): |startup_cost += indexTotalCost; But maybe it shouldn't (?) Or maybe it should take a third value, like enable_indexscan=bitmaponly, which means what it says. Actually the name is confusable with indexonly, so maybe enable_indexscan=bitmap. A third value isn't really needed anyway; its only utility is that someone upgrading from v12 who uses enable_indexscan=off (presumably in limited scope) wouldn't have to also set enable_bitmapscan=off - not a big benefit. That doesn't affect regress tests at all. Note, when I tested it, the cost of "bitmap heap scan" was several times higher than the total cost of indexscan (including heap), even with CPU costs at 0. I applied my "bitmap correlation" patch, which seems to gives more reasonable result. In any case, the purpose of this patch was primarily diagnostic, and the heap cost of index scan would be its total cost minus the cost of the bitmap indexscan node when enable_indexscan=off. The high cost attributed to bitmap heapscan is topic for the other patch. Justin
>From 6ad506879d8a754013b971197592fc9617850b7e Mon Sep 17 00:00:00 2001 From: Justin Pryzby <pryz...@telsasoft.com> Date: Sat, 4 Jan 2020 10:25:12 -0600 Subject: [PATCH v1] allow disabling indexscans but not bitmap scans --- src/backend/optimizer/path/costsize.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index b5a0033..6f37386 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -972,7 +972,12 @@ cost_bitmap_heap_scan(Path *path, PlannerInfo *root, RelOptInfo *baserel, loop_count, &indexTotalCost, &tuples_fetched); - startup_cost += indexTotalCost; + if (indexTotalCost > disable_cost && enable_bitmapscan) + /* enable_indexscan=off no longer itself disables bitmap scans */ + startup_cost += indexTotalCost - disable_cost; + else + startup_cost += indexTotalCost; + T = (baserel->pages > 1) ? (double) baserel->pages : 1.0; /* Fetch estimated page costs for tablespace containing table. */ -- 2.7.4
