Re: Regarding the necessity of RelationGetNumberOfBlocks for every rescan / bitmap heap scan.
| От | Andy Fan |
|---|---|
| Тема | Re: Regarding the necessity of RelationGetNumberOfBlocks for every rescan / bitmap heap scan. |
| Дата | |
| Msg-id | CAKU4AWr_AfHXtrzc8Gy9V3JD2m0SGXROMEQV=CJraRkFcZJFJg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Regarding the necessity of RelationGetNumberOfBlocks for every rescan / bitmap heap scan. (Andy Fan <zhihui.fan1213@gmail.com>) |
| Список | pgsql-hackers |
1. Why do we need scan->rs_nblocks =
RelationGextNumberOfBlocks(scan->rs_base.rs_rd) for every rescan, which looks
mismatched with the comments along the code. and the comments looks
reasonable to me.
2. For the heap scan after an IndexScan, we don't need to know the heap
size, then why do we need to get the nblocks for bitmap heap scan? I think the
similarity between the 2 is that both of them can get a "valid" CTID/pages number
from index scan. To be clearer, I think for bitmap heap scan, we even don't
need check the RelationGextNumberOfBlocks for the initscan.3. If we need to check nblocks every time, why Parallel Scan doesn't change itevery time?shall we remove the RelationGextNumberOfBlocks for bitmap heap scan totallyand the rescan for normal heap scan?
yizhi.fzh@e18c07352 /u/y/g/postgres> git diff
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 6ac07f2fda..6df096fb46 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -246,7 +246,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
bpscan = (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel;
scan->rs_nblocks = bpscan->phs_nblocks;
}
- else
+ else if (scan->rs_nblocks == -1 && !(scan->rs_base.rs_flags & SO_TYPE_BITMAPSCAN))
scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_base.rs_rd);
/*
@@ -1209,6 +1209,7 @@ heap_beginscan(Relation relation, Snapshot snapshot,
scan->rs_base.rs_flags = flags;
scan->rs_base.rs_parallel = parallel_scan;
scan->rs_strategy = NULL; /* set in initscan */
+ scan->rs_nblocks = -1;
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 6ac07f2fda..6df096fb46 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -246,7 +246,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
bpscan = (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel;
scan->rs_nblocks = bpscan->phs_nblocks;
}
- else
+ else if (scan->rs_nblocks == -1 && !(scan->rs_base.rs_flags & SO_TYPE_BITMAPSCAN))
scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_base.rs_rd);
/*
@@ -1209,6 +1209,7 @@ heap_beginscan(Relation relation, Snapshot snapshot,
scan->rs_base.rs_flags = flags;
scan->rs_base.rs_parallel = parallel_scan;
scan->rs_strategy = NULL; /* set in initscan */
+ scan->rs_nblocks = -1;
I did the above hacks, and all the existing tests passed.
Best Regards
Andy Fan (https://www.aliyun.com/)
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