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Account for the effect of lossy pages when costing bitmap scans.

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Dilip Kumar, reviewed by Alexander Kumenkov, Amul Sul, and me.
Some final adjustments by me.

Discussion: http://postgr.es/m/CAFiTN-sYtqUOXQ4SpuhTv0Z9gD0si3YxZGv_PQAAMX8qbOotcg@mail.gmail.com
This commit is contained in:
Robert Haas
2017-11-10 16:50:50 -05:00
parent 0c98d0dd5c
commit 5edc63bda6
3 changed files with 75 additions and 22 deletions
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37
src/backend/nodes/tidbitmap.c
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@@ -265,7 +265,6 @@ TIDBitmap *
tbm_create(long maxbytes, dsa_area *dsa)
{
TIDBitmap *tbm;
long nbuckets;
/* Create the TIDBitmap struct and zero all its fields */
tbm = makeNode(TIDBitmap);
@@ -273,17 +272,7 @@ tbm_create(long maxbytes, dsa_area *dsa)
tbm->mcxt = CurrentMemoryContext;
tbm->status = TBM_EMPTY;
/*
* Estimate number of hashtable entries we can have within maxbytes. This
* estimates the hash cost as sizeof(PagetableEntry), which is good enough
* for our purpose. Also count an extra Pointer per entry for the arrays
* created during iteration readout.
*/
nbuckets = maxbytes /
(sizeof(PagetableEntry) + sizeof(Pointer) + sizeof(Pointer));
nbuckets = Min(nbuckets, INT_MAX - 1); /* safety limit */
nbuckets = Max(nbuckets, 16); /* sanity limit */
tbm->maxentries = (int) nbuckets;
tbm->maxentries = (int) tbm_calculate_entries(maxbytes);
tbm->lossify_start = 0;
tbm->dsa = dsa;
tbm->dsapagetable = InvalidDsaPointer;
@@ -1546,3 +1535,27 @@ pagetable_free(pagetable_hash *pagetable, void *pointer)
tbm->dsapagetableold = InvalidDsaPointer;
}
}
/*
* tbm_calculate_entries
*
* Estimate number of hashtable entries we can have within maxbytes.
*/
long
tbm_calculate_entries(double maxbytes)
{
long nbuckets;
/*
* Estimate number of hashtable entries we can have within maxbytes. This
* estimates the hash cost as sizeof(PagetableEntry), which is good enough
* for our purpose. Also count an extra Pointer per entry for the arrays
* created during iteration readout.
*/
nbuckets = maxbytes /
(sizeof(PagetableEntry) + sizeof(Pointer) + sizeof(Pointer));
nbuckets = Min(nbuckets, INT_MAX - 1); /* safety limit */
nbuckets = Max(nbuckets, 16); /* sanity limit */
return nbuckets;
}