I wrote:
> So what this seems to mean is that for SEMI/ANTI join cases, we have to
> postpone all of the inner scan cost determination to final_cost_nestloop,
> so that we can do this differently depending on whether
> has_indexed_join_quals() is true. That's a little bit annoying because it
> will mean we take the shortcut exit less often; but since SEMI/ANTI joins
> aren't that common, it's probably not going to be a big planning time hit.
Attached is a draft patch for that. It fixes the problem for me:
Nested Loop Semi Join (cost=0.99..9.09 rows=1 width=74) (actual
time=0.591..1.554 rows=2 loops=1)
-> Index Scan using term_facttablename_columnname_idx on term t
(cost=0.55..8.57 rows=1 width=74) (actual time=0.022..0.025 rows=2 loops=1)
Index Cond: (((facttablename)::text = 'facttable_stat_fta4'::text) AND
((columnname)::text = 'berechnungsart'::text))
-> Index Only Scan using facttable_stat_fta4_berechnungsart_idx on
facttable_stat_fta4 f (cost=0.43..143244.98 rows=5015134 width=2) (actual
time=0.759..0.759 rows=1 loops=2)
Index Cond: (berechnungsart = (t.term)::text)
Heap Fetches: 0
Planning time: 0.545 ms
Execution time: 1.615 ms
> Not sure yet about your other point about the indexscan getting rejected
> too soon. That doesn't seem to be happening for me, at least not in HEAD.
I do see something of the sort if I turn off enable_indexonlyscan.
Not sure about a good fix for that aspect. It may not be terribly
critical, since AFAICS index-only scans generally ought to apply
in these cases.
regards, tom lane
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index ac865be..0d302f6 100644
*** a/src/backend/optimizer/path/costsize.c
--- b/src/backend/optimizer/path/costsize.c
*************** cost_group(Path *path, PlannerInfo *root
*** 1767,1773 ****
* estimate and getting a tight lower bound. We choose to not examine the
* join quals here, since that's by far the most expensive part of the
* calculations. The end result is that CPU-cost considerations must be
! * left for the second phase.
*
* 'workspace' is to be filled with startup_cost, total_cost, and perhaps
* other data to be used by final_cost_nestloop
--- 1767,1774 ----
* estimate and getting a tight lower bound. We choose to not examine the
* join quals here, since that's by far the most expensive part of the
* calculations. The end result is that CPU-cost considerations must be
! * left for the second phase; and for SEMI/ANTI joins, we must also postpone
! * incorporation of the inner path's run cost.
*
* 'workspace' is to be filled with startup_cost, total_cost, and perhaps
* other data to be used by final_cost_nestloop
*************** initial_cost_nestloop(PlannerInfo *root,
*** 1815,1858 ****
if (jointype == JOIN_SEMI || jointype == JOIN_ANTI)
{
- double outer_matched_rows;
- Selectivity inner_scan_frac;
-
/*
* SEMI or ANTI join: executor will stop after first match.
*
! * For an outer-rel row that has at least one match, we can expect the
! * inner scan to stop after a fraction 1/(match_count+1) of the inner
! * rows, if the matches are evenly distributed. Since they probably
! * aren't quite evenly distributed, we apply a fuzz factor of 2.0 to
! * that fraction. (If we used a larger fuzz factor, we'd have to
! * clamp inner_scan_frac to at most 1.0; but since match_count is at
! * least 1, no such clamp is needed now.)
! *
! * A complicating factor is that rescans may be cheaper than first
! * scans. If we never scan all the way to the end of the inner rel,
! * it might be (depending on the plan type) that we'd never pay the
! * whole inner first-scan run cost. However it is difficult to
! * estimate whether that will happen, so be conservative and always
! * charge the whole first-scan cost once.
! */
! run_cost += inner_run_cost;
!
! outer_matched_rows = rint(outer_path_rows * semifactors->outer_match_frac);
! inner_scan_frac = 2.0 / (semifactors->match_count + 1.0);
!
! /* Add inner run cost for additional outer tuples having matches */
! if (outer_matched_rows > 1)
! run_cost += (outer_matched_rows - 1) * inner_rescan_run_cost * inner_scan_frac;
!
! /*
! * The cost of processing unmatched rows varies depending on the
! * details of the joinclauses, so we leave that part for later.
*/
/* Save private data for final_cost_nestloop */
! workspace->outer_matched_rows = outer_matched_rows;
! workspace->inner_scan_frac = inner_scan_frac;
}
else
{
--- 1816,1831 ----
if (jointype == JOIN_SEMI || jointype == JOIN_ANTI)
{
/*
* SEMI or ANTI join: executor will stop after first match.
*
! * Getting decent estimates requires inspection of the join quals,
! * which we choose to postpone to final_cost_nestloop.
*/
/* Save private data for final_cost_nestloop */
! workspace->inner_run_cost = inner_run_cost;
! workspace->inner_rescan_run_cost = inner_rescan_run_cost;
}
else
{
*************** initial_cost_nestloop(PlannerInfo *root,
*** 1869,1875 ****
workspace->total_cost = startup_cost + run_cost;
/* Save private data for final_cost_nestloop */
workspace->run_cost = run_cost;
- workspace->inner_rescan_run_cost = inner_rescan_run_cost;
}
/*
--- 1842,1847 ----
*************** final_cost_nestloop(PlannerInfo *root, N
*** 1893,1899 ****
double inner_path_rows = inner_path->rows;
Cost startup_cost = workspace->startup_cost;
Cost run_cost = workspace->run_cost;
- Cost inner_rescan_run_cost = workspace->inner_rescan_run_cost;
Cost cpu_per_tuple;
QualCost restrict_qual_cost;
double ntuples;
--- 1865,1870 ----
*************** final_cost_nestloop(PlannerInfo *root, N
*** 1912,1953 ****
if (!enable_nestloop)
startup_cost += disable_cost;
! /* cost of source data */
if (path->jointype == JOIN_SEMI || path->jointype == JOIN_ANTI)
{
- double outer_matched_rows = workspace->outer_matched_rows;
- Selectivity inner_scan_frac = workspace->inner_scan_frac;
-
/*
* SEMI or ANTI join: executor will stop after first match.
*/
! /* Compute number of tuples processed (not number emitted!) */
ntuples = outer_matched_rows * inner_path_rows * inner_scan_frac;
/*
! * For unmatched outer-rel rows, there are two cases. If the inner
! * path is an indexscan using all the joinquals as indexquals, then an
! * unmatched row results in an indexscan returning no rows, which is
! * probably quite cheap. We estimate this case as the same cost to
! * return the first tuple of a nonempty scan. Otherwise, the executor
! * will have to scan the whole inner rel; not so cheap.
*/
if (has_indexed_join_quals(path))
{
run_cost += (outer_path_rows - outer_matched_rows) *
inner_rescan_run_cost / inner_path_rows;
/*
! * We won't be evaluating any quals at all for these rows, so
* don't add them to ntuples.
*/
}
else
{
run_cost += (outer_path_rows - outer_matched_rows) *
inner_rescan_run_cost;
ntuples += (outer_path_rows - outer_matched_rows) *
inner_path_rows;
}
--- 1883,1983 ----
if (!enable_nestloop)
startup_cost += disable_cost;
! /* cost of inner-relation source data (we already dealt with outer rel) */
if (path->jointype == JOIN_SEMI || path->jointype == JOIN_ANTI)
{
/*
* SEMI or ANTI join: executor will stop after first match.
*/
+ Cost inner_run_cost = workspace->inner_run_cost;
+ Cost inner_rescan_run_cost = workspace->inner_rescan_run_cost;
+ double outer_matched_rows;
+ Selectivity inner_scan_frac;
! /*
! * For an outer-rel row that has at least one match, we can expect the
! * inner scan to stop after a fraction 1/(match_count+1) of the inner
! * rows, if the matches are evenly distributed. Since they probably
! * aren't quite evenly distributed, we apply a fuzz factor of 2.0 to
! * that fraction. (If we used a larger fuzz factor, we'd have to
! * clamp inner_scan_frac to at most 1.0; but since match_count is at
! * least 1, no such clamp is needed now.)
! */
! outer_matched_rows = rint(outer_path_rows * semifactors->outer_match_frac);
! inner_scan_frac = 2.0 / (semifactors->match_count + 1.0);
!
! /*
! * Compute number of tuples processed (not number emitted!). First,
! * account for successfully-matched outer rows.
! */
ntuples = outer_matched_rows * inner_path_rows * inner_scan_frac;
/*
! * Now we need to estimate the actual costs of scanning the inner
! * relation, which may be quite a bit less than N times inner_run_cost
! * due to early scan stops. We consider two cases. If the inner path
! * is an indexscan using all the joinquals as indexquals, then an
! * unmatched outer row results in an indexscan returning no rows,
! * which is probably quite cheap. Otherwise, the executor will have
! * to scan the whole inner rel for an unmatched row; not so cheap.
*/
if (has_indexed_join_quals(path))
{
+ /*
+ * Successfully-matched outer rows will only require scanning
+ * inner_scan_frac of the inner relation. In this case, we don't
+ * need to charge the full inner_run_cost even when that's more
+ * than inner_rescan_run_cost, because we can assume that none of
+ * the inner scans ever scan the whole inner relation. So it's
+ * okay to assume that all the inner scan executions can be
+ * fractions of the full cost, even if materialization is reducing
+ * the rescan cost. At this writing, it's impossible to get here
+ * for a materialized inner scan, so inner_run_cost and
+ * inner_rescan_run_cost will be the same anyway; but just in
+ * case, use inner_run_cost for the first matched tuple and
+ * inner_rescan_run_cost for additional ones.
+ */
+ run_cost += inner_run_cost * inner_scan_frac;
+ if (outer_matched_rows > 1)
+ run_cost += (outer_matched_rows - 1) * inner_rescan_run_cost * inner_scan_frac;
+
+ /*
+ * Add the cost of inner-scan executions for unmatched outer rows.
+ * We estimate this as the same cost as returning the first tuple
+ * of a nonempty scan. We consider that these are all rescans,
+ * since we used inner_run_cost once already.
+ */
run_cost += (outer_path_rows - outer_matched_rows) *
inner_rescan_run_cost / inner_path_rows;
/*
! * We won't be evaluating any quals at all for unmatched rows, so
* don't add them to ntuples.
*/
}
else
{
+ /*
+ * Here, a complicating factor is that rescans may be cheaper than
+ * first scans. If we never scan all the way to the end of the
+ * inner rel, it might be (depending on the plan type) that we'd
+ * never pay the whole inner first-scan run cost. However it is
+ * difficult to estimate whether that will happen (and it could
+ * not happen if there are any unmatched outer rows!), so be
+ * conservative and always charge the whole first-scan cost once.
+ */
+ run_cost += inner_run_cost;
+
+ /* Add inner run cost for additional outer tuples having matches */
+ if (outer_matched_rows > 1)
+ run_cost += (outer_matched_rows - 1) * inner_rescan_run_cost * inner_scan_frac;
+
+ /* Add inner run cost for unmatched outer tuples */
run_cost += (outer_path_rows - outer_matched_rows) *
inner_rescan_run_cost;
+
+ /* And count the unmatched join tuples as being processed */
ntuples += (outer_path_rows - outer_matched_rows) *
inner_path_rows;
}
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 279051e..706b6d1 100644
*** a/src/include/nodes/relation.h
--- b/src/include/nodes/relation.h
*************** typedef struct JoinCostWorkspace
*** 1719,1730 ****
Cost run_cost; /* non-startup cost components */
/* private for cost_nestloop code */
Cost inner_rescan_run_cost;
- double outer_matched_rows;
- Selectivity inner_scan_frac;
/* private for cost_mergejoin code */
- Cost inner_run_cost;
double outer_rows;
double inner_rows;
double outer_skip_rows;
--- 1719,1728 ----
Cost run_cost; /* non-startup cost components */
/* private for cost_nestloop code */
+ Cost inner_run_cost; /* also used by cost_mergejoin code */
Cost inner_rescan_run_cost;
/* private for cost_mergejoin code */
double outer_rows;
double inner_rows;
double outer_skip_rows;
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