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Commit9391f71

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Teach estimate_array_length() to use statistics where available.
If we have DECHIST statistics about the argument expression, usethe average number of distinct elements as the array length estimate.(It'd be better to use the average total number of elements, butthat is not currently calculated by compute_array_stats(), andit's unclear that it'd be worth extra effort to get.)To do this, we have to change the signature of estimate_array_lengthto pass the "root" pointer. While at it, also change its resulttype to "double". That's probably not really necessary, but itavoids any risk of overflow of the value extracted from DECHIST.All existing callers are going to use the result in a "double"calculation anyway.Paul Jungwirth, reviewed by Jian He and myselfDiscussion:https://postgr.es/m/CA+renyUnM2d+SmrxKpDuAdpiq6FOM=FByvi6aS6yi__qyf6j9A@mail.gmail.com
1 parent14dd0f2 commit9391f71

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4 files changed

+45
-16
lines changed

4 files changed

+45
-16
lines changed

‎src/backend/optimizer/path/costsize.c

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1256,7 +1256,7 @@ cost_tidscan(Path *path, PlannerInfo *root,
12561256
QualCostqpqual_cost;
12571257
Costcpu_per_tuple;
12581258
QualCosttid_qual_cost;
1259-
intntuples;
1259+
doublentuples;
12601260
ListCell*l;
12611261
doublespc_random_page_cost;
12621262

@@ -1283,7 +1283,7 @@ cost_tidscan(Path *path, PlannerInfo *root,
12831283
ScalarArrayOpExpr*saop= (ScalarArrayOpExpr*)qual;
12841284
Node*arraynode= (Node*)lsecond(saop->args);
12851285

1286-
ntuples+=estimate_array_length(arraynode);
1286+
ntuples+=estimate_array_length(root,arraynode);
12871287
}
12881288
elseif (IsA(qual,CurrentOfExpr))
12891289
{
@@ -4770,7 +4770,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
47704770
Node*arraynode= (Node*)lsecond(saop->args);
47714771
QualCostsacosts;
47724772
QualCosthcosts;
4773-
intestarraylen=estimate_array_length(arraynode);
4773+
doubleestarraylen=estimate_array_length(context->root,arraynode);
47744774

47754775
set_sa_opfuncid(saop);
47764776
sacosts.startup=sacosts.per_tuple=0;
@@ -4808,7 +4808,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
48084808
*/
48094809
context->total.startup+=sacosts.startup;
48104810
context->total.per_tuple+=sacosts.per_tuple*
4811-
estimate_array_length(arraynode)*0.5;
4811+
estimate_array_length(context->root,arraynode)*0.5;
48124812
}
48134813
}
48144814
elseif (IsA(node,Aggref)||
@@ -4859,7 +4859,7 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
48594859
context->total.startup+=perelemcost.startup;
48604860
if (perelemcost.per_tuple>0)
48614861
context->total.per_tuple+=perelemcost.per_tuple*
4862-
estimate_array_length((Node*)acoerce->arg);
4862+
estimate_array_length(context->root,(Node*)acoerce->arg);
48634863
}
48644864
elseif (IsA(node,RowCompareExpr))
48654865
{

‎src/backend/utils/adt/arrayfuncs.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -6340,7 +6340,7 @@ array_unnest_support(PG_FUNCTION_ARGS)
63406340
/* We can use estimated argument values here */
63416341
arg1=estimate_expression_value(req->root,linitial(args));
63426342

6343-
req->rows=estimate_array_length(arg1);
6343+
req->rows=estimate_array_length(req->root,arg1);
63446344
ret= (Node*)req;
63456345
}
63466346
}

‎src/backend/utils/adt/selfuncs.c

Lines changed: 38 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -2128,10 +2128,11 @@ scalararraysel(PlannerInfo *root,
21282128
/*
21292129
* Estimate number of elements in the array yielded by an expression.
21302130
*
2131-
* It's important that this agree with scalararraysel.
2131+
* Note: the result is integral, but we use "double" to avoid overflow
2132+
* concerns. Most callers will use it in double-type expressions anyway.
21322133
*/
2133-
int
2134-
estimate_array_length(Node*arrayexpr)
2134+
double
2135+
estimate_array_length(PlannerInfo*root,Node*arrayexpr)
21352136
{
21362137
/* look through any binary-compatible relabeling of arrayexpr */
21372138
arrayexpr=strip_array_coercion(arrayexpr);
@@ -2152,11 +2153,39 @@ estimate_array_length(Node *arrayexpr)
21522153
{
21532154
returnlist_length(((ArrayExpr*)arrayexpr)->elements);
21542155
}
2155-
else
2156+
elseif (arrayexpr)
21562157
{
2157-
/* default guess --- see also scalararraysel */
2158-
return10;
2158+
/* See if we can find any statistics about it */
2159+
VariableStatDatavardata;
2160+
AttStatsSlotsslot;
2161+
doublenelem=0;
2162+
2163+
examine_variable(root,arrayexpr,0,&vardata);
2164+
if (HeapTupleIsValid(vardata.statsTuple))
2165+
{
2166+
/*
2167+
* Found stats, so use the average element count, which is stored
2168+
* in the last stanumbers element of the DECHIST statistics.
2169+
* Actually that is the average count of *distinct* elements;
2170+
* perhaps we should scale it up somewhat?
2171+
*/
2172+
if (get_attstatsslot(&sslot,vardata.statsTuple,
2173+
STATISTIC_KIND_DECHIST,InvalidOid,
2174+
ATTSTATSSLOT_NUMBERS))
2175+
{
2176+
if (sslot.nnumbers>0)
2177+
nelem=clamp_row_est(sslot.numbers[sslot.nnumbers-1]);
2178+
free_attstatsslot(&sslot);
2179+
}
2180+
}
2181+
ReleaseVariableStats(vardata);
2182+
2183+
if (nelem>0)
2184+
returnnelem;
21592185
}
2186+
2187+
/* Else use a default guess --- this should match scalararraysel */
2188+
return10;
21602189
}
21612190

21622191
/*
@@ -6540,7 +6569,7 @@ genericcostestimate(PlannerInfo *root,
65406569
if (IsA(rinfo->clause,ScalarArrayOpExpr))
65416570
{
65426571
ScalarArrayOpExpr*saop= (ScalarArrayOpExpr*)rinfo->clause;
6543-
intalength=estimate_array_length(lsecond(saop->args));
6572+
doublealength=estimate_array_length(root,lsecond(saop->args));
65446573

65456574
if (alength>1)
65466575
num_sa_scans *=alength;
@@ -6820,7 +6849,7 @@ btcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
68206849
{
68216850
ScalarArrayOpExpr*saop= (ScalarArrayOpExpr*)clause;
68226851
Node*other_operand= (Node*)lsecond(saop->args);
6823-
intalength=estimate_array_length(other_operand);
6852+
doublealength=estimate_array_length(root,other_operand);
68246853

68256854
clause_op=saop->opno;
68266855
found_saop= true;
@@ -7414,7 +7443,7 @@ gincost_scalararrayopexpr(PlannerInfo *root,
74147443
{
74157444
counts->exactEntries++;
74167445
counts->searchEntries++;
7417-
counts->arrayScans *=estimate_array_length(rightop);
7446+
counts->arrayScans *=estimate_array_length(root,rightop);
74187447
return true;
74197448
}
74207449

‎src/include/utils/selfuncs.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -200,7 +200,7 @@ extern Selectivity scalararraysel(PlannerInfo *root,
200200
ScalarArrayOpExpr*clause,
201201
boolis_join_clause,
202202
intvarRelid,JoinTypejointype,SpecialJoinInfo*sjinfo);
203-
externintestimate_array_length(Node*arrayexpr);
203+
externdoubleestimate_array_length(PlannerInfo*root,Node*arrayexpr);
204204
externSelectivityrowcomparesel(PlannerInfo*root,
205205
RowCompareExpr*clause,
206206
intvarRelid,JoinTypejointype,SpecialJoinInfo*sjinfo);

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