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Postpone calculating total_table_pages until after pruning/exclusion.

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The planner doesn't make any use of root->total_table_pages until it
estimates costs of indexscans, so we don't need to compute it as
early as that's currently done.  By doing the calculation between
set_base_rel_sizes and set_base_rel_pathlists, we can omit relations
that get removed from the query by partition pruning or constraint
exclusion, which seems like a more accurate basis for costing.

(Historical note: I think at the time this code was written, there
was not a separation between the "set sizes" and "set pathlists"
steps, so that this approach would have been impossible at the time.
But now that we have that separation, this is clearly the better way
to do things.)

David Rowley, reviewed by Edmund Horner

Discussion: https://postgr.es/m/CAKJS1f-NG1mRM0VOtkAG7=ZLQWihoqees9R4ki3CKBB0-fRfCA@mail.gmail.com
This commit is contained in:
Tom Lane
2018-11-07 12:12:56 -05:00
parent 77366d90f4
commit c6e4133fae
4 changed files with 49 additions and 39 deletions
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45
src/backend/optimizer/path/allpaths.c
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@ -147,6 +147,7 @@ make_one_rel(PlannerInfo *root, List *joinlist)
{
RelOptInfo *rel;
Index rti;
double total_pages;
/*
* Construct the all_baserels Relids set.
@ -173,10 +174,45 @@ make_one_rel(PlannerInfo *root, List *joinlist)
set_base_rel_consider_startup(root);
/*
* Compute size estimates and consider_parallel flags for each base rel,
* then generate access paths.
* Compute size estimates and consider_parallel flags for each base rel.
*/
set_base_rel_sizes(root);
/*
* We should now have size estimates for every actual table involved in
* the query, and we also know which if any have been deleted from the
* query by join removal, pruned by partition pruning, or eliminated by
* constraint exclusion. So we can now compute total_table_pages.
*
* Note that appendrels are not double-counted here, even though we don't
* bother to distinguish RelOptInfos for appendrel parents, because the
* parents will have pages = 0.
*
* XXX if a table is self-joined, we will count it once per appearance,
* which perhaps is the wrong thing ... but that's not completely clear,
* and detecting self-joins here is difficult, so ignore it for now.
*/
total_pages = 0;
for (rti = 1; rti < root->simple_rel_array_size; rti++)
{
RelOptInfo *brel = root->simple_rel_array[rti];
if (brel == NULL)
continue;
Assert(brel->relid == rti); /* sanity check on array */
if (IS_DUMMY_REL(brel))
continue;
if (IS_SIMPLE_REL(brel))
total_pages += (double) brel->pages;
}
root->total_table_pages = total_pages;
/*
* Generate access paths for each base rel.
*/
set_base_rel_pathlists(root);
/*
@ -1271,6 +1307,11 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
* because some places assume rel->tuples is valid for any baserel.
*/
rel->tuples = parent_rows;
/*
* Note that we leave rel->pages as zero; this is important to avoid
* double-counting the appendrel tree in total_table_pages.
*/
}
else
{