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Commitb557226

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Improve planner's cost estimation in the presence of semijoins.
If we have a semijoin, saySELECT * FROM x WHERE x1 IN (SELECT y1 FROM y)and we're estimating the cost of a parameterized indexscan on x, the numberof repetitions of the indexscan should not be taken as the size of y; it'llreally only be the number of distinct values of y1, because the only validplan with y on the outside of a nestloop would require y to be unique-ifiedbefore joining it to x. Most of the time this doesn't make that muchdifference, but sometimes it can lead to drastically underestimating thecost of the indexscan and hence choosing a bad plan, as pointed out byDavid Kubečka.Fixing this is a bit difficult because parameterized indexscans are costedout quite early in the planning process, before we have the informationthat would be needed to call estimate_num_groups() and thereby estimate thenumber of distinct values of the join column(s). However we can move thecode that extracts a semijoin RHS's unique-ification columns, so that it'sdone in initsplan.c rather than on-the-fly in create_unique_path(). Thatshouldn't make any difference speed-wise and it's really a bit cleaner too.The other bit of information we need is the size of the semijoin RHS,which is easy if it's a single relation (we make those estimates beforeconsidering indexscan costs) but problematic if it's a join relation.The solution adopted here is just to use the product of the sizes of thejoin component rels. That will generally be an overestimate, but sinceestimate_num_groups() only uses this input as a clamp, an overestimateshouldn't hurt us too badly. In any case we don't allow this new logicto produce a value larger than we would have chosen before, so that atworst an overestimate leaves us no wiser than we were before.
1 parentff2faee commitb557226

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10 files changed

+385
-224
lines changed

10 files changed

+385
-224
lines changed

‎src/backend/nodes/copyfuncs.c‎

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1942,7 +1942,10 @@ _copySpecialJoinInfo(const SpecialJoinInfo *from)
19421942
COPY_SCALAR_FIELD(jointype);
19431943
COPY_SCALAR_FIELD(lhs_strict);
19441944
COPY_SCALAR_FIELD(delay_upper_joins);
1945-
COPY_NODE_FIELD(join_quals);
1945+
COPY_SCALAR_FIELD(semi_can_btree);
1946+
COPY_SCALAR_FIELD(semi_can_hash);
1947+
COPY_NODE_FIELD(semi_operators);
1948+
COPY_NODE_FIELD(semi_rhs_exprs);
19461949

19471950
returnnewnode;
19481951
}

‎src/backend/nodes/equalfuncs.c‎

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -798,7 +798,10 @@ _equalSpecialJoinInfo(const SpecialJoinInfo *a, const SpecialJoinInfo *b)
798798
COMPARE_SCALAR_FIELD(jointype);
799799
COMPARE_SCALAR_FIELD(lhs_strict);
800800
COMPARE_SCALAR_FIELD(delay_upper_joins);
801-
COMPARE_NODE_FIELD(join_quals);
801+
COMPARE_SCALAR_FIELD(semi_can_btree);
802+
COMPARE_SCALAR_FIELD(semi_can_hash);
803+
COMPARE_NODE_FIELD(semi_operators);
804+
COMPARE_NODE_FIELD(semi_rhs_exprs);
802805

803806
return true;
804807
}

‎src/backend/nodes/outfuncs.c‎

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1945,7 +1945,10 @@ _outSpecialJoinInfo(StringInfo str, const SpecialJoinInfo *node)
19451945
WRITE_ENUM_FIELD(jointype,JoinType);
19461946
WRITE_BOOL_FIELD(lhs_strict);
19471947
WRITE_BOOL_FIELD(delay_upper_joins);
1948-
WRITE_NODE_FIELD(join_quals);
1948+
WRITE_BOOL_FIELD(semi_can_btree);
1949+
WRITE_BOOL_FIELD(semi_can_hash);
1950+
WRITE_NODE_FIELD(semi_operators);
1951+
WRITE_NODE_FIELD(semi_rhs_exprs);
19491952
}
19501953

19511954
staticvoid

‎src/backend/optimizer/path/costsize.c‎

Lines changed: 8 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -3294,7 +3294,10 @@ compute_semi_anti_join_factors(PlannerInfo *root,
32943294
/* we don't bother trying to make the remaining fields valid */
32953295
norm_sjinfo.lhs_strict= false;
32963296
norm_sjinfo.delay_upper_joins= false;
3297-
norm_sjinfo.join_quals=NIL;
3297+
norm_sjinfo.semi_can_btree= false;
3298+
norm_sjinfo.semi_can_hash= false;
3299+
norm_sjinfo.semi_operators=NIL;
3300+
norm_sjinfo.semi_rhs_exprs=NIL;
32983301

32993302
nselec=clauselist_selectivity(root,
33003303
joinquals,
@@ -3456,7 +3459,10 @@ approx_tuple_count(PlannerInfo *root, JoinPath *path, List *quals)
34563459
/* we don't bother trying to make the remaining fields valid */
34573460
sjinfo.lhs_strict= false;
34583461
sjinfo.delay_upper_joins= false;
3459-
sjinfo.join_quals=NIL;
3462+
sjinfo.semi_can_btree= false;
3463+
sjinfo.semi_can_hash= false;
3464+
sjinfo.semi_operators=NIL;
3465+
sjinfo.semi_rhs_exprs=NIL;
34603466

34613467
/* Get the approximate selectivity */
34623468
foreach(l,quals)

‎src/backend/optimizer/path/indxpath.c‎

Lines changed: 128 additions & 29 deletions
Original file line numberDiff line numberDiff line change
@@ -130,7 +130,12 @@ static Relids get_bitmap_tree_required_outer(Path *bitmapqual);
130130
staticvoidfind_indexpath_quals(Path*bitmapqual,List**quals,List**preds);
131131
staticintfind_list_position(Node*node,List**nodelist);
132132
staticboolcheck_index_only(RelOptInfo*rel,IndexOptInfo*index);
133-
staticdoubleget_loop_count(PlannerInfo*root,Relidsouter_relids);
133+
staticdoubleget_loop_count(PlannerInfo*root,Indexcur_relid,Relidsouter_relids);
134+
staticdoubleadjust_rowcount_for_semijoins(PlannerInfo*root,
135+
Indexcur_relid,
136+
Indexouter_relid,
137+
doublerowcount);
138+
staticdoubleapproximate_joinrel_size(PlannerInfo*root,Relidsrelids);
134139
staticvoidmatch_restriction_clauses_to_index(RelOptInfo*rel,
135140
IndexOptInfo*index,
136141
IndexClauseSet*clauseset);
@@ -402,7 +407,7 @@ create_index_paths(PlannerInfo *root, RelOptInfo *rel)
402407

403408
/* And push that path into the mix */
404409
required_outer=get_bitmap_tree_required_outer(bitmapqual);
405-
loop_count=get_loop_count(root,required_outer);
410+
loop_count=get_loop_count(root,rel->relid,required_outer);
406411
bpath=create_bitmap_heap_path(root,rel,bitmapqual,
407412
required_outer,loop_count);
408413
add_path(rel, (Path*)bpath);
@@ -969,7 +974,7 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
969974
outer_relids=NULL;
970975

971976
/* Compute loop_count for cost estimation purposes */
972-
loop_count=get_loop_count(root,outer_relids);
977+
loop_count=get_loop_count(root,rel->relid,outer_relids);
973978

974979
/*
975980
* 2. Compute pathkeys describing index's ordering, if any, then see how
@@ -1553,7 +1558,7 @@ bitmap_scan_cost_est(PlannerInfo *root, RelOptInfo *rel, Path *ipath)
15531558
cost_bitmap_heap_scan(&bpath.path,root,rel,
15541559
bpath.path.param_info,
15551560
ipath,
1556-
get_loop_count(root,required_outer));
1561+
get_loop_count(root,rel->relid,required_outer));
15571562

15581563
returnbpath.path.total_cost;
15591564
}
@@ -1594,7 +1599,7 @@ bitmap_and_cost_est(PlannerInfo *root, RelOptInfo *rel, List *paths)
15941599
cost_bitmap_heap_scan(&bpath.path,root,rel,
15951600
bpath.path.param_info,
15961601
(Path*)&apath,
1597-
get_loop_count(root,required_outer));
1602+
get_loop_count(root,rel->relid,required_outer));
15981603

15991604
returnbpath.path.total_cost;
16001605
}
@@ -1861,48 +1866,142 @@ check_index_only(RelOptInfo *rel, IndexOptInfo *index)
18611866
* answer for single-other-relation cases, and it seems like a reasonable
18621867
* zero-order approximation for multiway-join cases.
18631868
*
1869+
* In addition, we check to see if the other side of each join clause is on
1870+
* the inside of some semijoin that the current relation is on the outside of.
1871+
* If so, the only way that a parameterized path could be used is if the
1872+
* semijoin RHS has been unique-ified, so we should use the number of unique
1873+
* RHS rows rather than using the relation's raw rowcount.
1874+
*
18641875
* Note: for this to work, allpaths.c must establish all baserel size
18651876
* estimates before it begins to compute paths, or at least before it
18661877
* calls create_index_paths().
18671878
*/
18681879
staticdouble
1869-
get_loop_count(PlannerInfo*root,Relidsouter_relids)
1880+
get_loop_count(PlannerInfo*root,Indexcur_relid,Relidsouter_relids)
18701881
{
1871-
doubleresult=1.0;
1882+
doubleresult;
1883+
intouter_relid;
18721884

18731885
/* For a non-parameterized path, just return 1.0 quickly */
1874-
if (outer_relids!=NULL)
1886+
if (outer_relids==NULL)
1887+
return1.0;
1888+
1889+
result=1.0;
1890+
outer_relid=-1;
1891+
while ((outer_relid=bms_next_member(outer_relids,outer_relid)) >=0)
18751892
{
1876-
intrelid;
1893+
RelOptInfo*outer_rel;
1894+
doublerowcount;
18771895

1878-
relid=-1;
1879-
while ((relid=bms_next_member(outer_relids,relid)) >=0)
1880-
{
1881-
RelOptInfo*outer_rel;
1896+
/* Paranoia: ignore bogus relid indexes */
1897+
if (outer_relid >=root->simple_rel_array_size)
1898+
continue;
1899+
outer_rel=root->simple_rel_array[outer_relid];
1900+
if (outer_rel==NULL)
1901+
continue;
1902+
Assert(outer_rel->relid==outer_relid);/* sanity check on array */
18821903

1883-
/* Paranoia: ignore bogus relid indexes */
1884-
if (relid >=root->simple_rel_array_size)
1885-
continue;
1886-
outer_rel=root->simple_rel_array[relid];
1887-
if (outer_rel==NULL)
1888-
continue;
1889-
Assert(outer_rel->relid==relid);/* sanity check on array */
1904+
/* Other relation could be proven empty, if so ignore */
1905+
if (IS_DUMMY_REL(outer_rel))
1906+
continue;
18901907

1891-
/* Other relation could be proven empty, if so ignore */
1892-
if (IS_DUMMY_REL(outer_rel))
1893-
continue;
1908+
/* Otherwise, rel's rows estimate should be valid by now */
1909+
Assert(outer_rel->rows>0);
18941910

1895-
/* Otherwise, rel's rows estimate should be valid by now */
1896-
Assert(outer_rel->rows>0);
1911+
/* Check to see if rel is on the inside of any semijoins */
1912+
rowcount=adjust_rowcount_for_semijoins(root,
1913+
cur_relid,
1914+
outer_relid,
1915+
outer_rel->rows);
18971916

1898-
/* Remember smallest row count estimate among the outer rels */
1899-
if (result==1.0||result>outer_rel->rows)
1900-
result=outer_rel->rows;
1901-
}
1917+
/* Remember smallest row count estimate among the outer rels */
1918+
if (result==1.0||result>rowcount)
1919+
result=rowcount;
19021920
}
19031921
returnresult;
19041922
}
19051923

1924+
/*
1925+
* Check to see if outer_relid is on the inside of any semijoin that cur_relid
1926+
* is on the outside of. If so, replace rowcount with the estimated number of
1927+
* unique rows from the semijoin RHS (assuming that's smaller, which it might
1928+
* not be). The estimate is crude but it's the best we can do at this stage
1929+
* of the proceedings.
1930+
*/
1931+
staticdouble
1932+
adjust_rowcount_for_semijoins(PlannerInfo*root,
1933+
Indexcur_relid,
1934+
Indexouter_relid,
1935+
doublerowcount)
1936+
{
1937+
ListCell*lc;
1938+
1939+
foreach(lc,root->join_info_list)
1940+
{
1941+
SpecialJoinInfo*sjinfo= (SpecialJoinInfo*)lfirst(lc);
1942+
1943+
if (sjinfo->jointype==JOIN_SEMI&&
1944+
bms_is_member(cur_relid,sjinfo->syn_lefthand)&&
1945+
bms_is_member(outer_relid,sjinfo->syn_righthand))
1946+
{
1947+
/* Estimate number of unique-ified rows */
1948+
doublenraw;
1949+
doublenunique;
1950+
1951+
nraw=approximate_joinrel_size(root,sjinfo->syn_righthand);
1952+
nunique=estimate_num_groups(root,
1953+
sjinfo->semi_rhs_exprs,
1954+
nraw);
1955+
if (rowcount>nunique)
1956+
rowcount=nunique;
1957+
}
1958+
}
1959+
returnrowcount;
1960+
}
1961+
1962+
/*
1963+
* Make an approximate estimate of the size of a joinrel.
1964+
*
1965+
* We don't have enough info at this point to get a good estimate, so we
1966+
* just multiply the base relation sizes together. Fortunately, this is
1967+
* the right answer anyway for the most common case with a single relation
1968+
* on the RHS of a semijoin. Also, estimate_num_groups() has only a weak
1969+
* dependency on its input_rows argument (it basically uses it as a clamp).
1970+
* So we might be able to get a fairly decent end result even with a severe
1971+
* overestimate of the RHS's raw size.
1972+
*/
1973+
staticdouble
1974+
approximate_joinrel_size(PlannerInfo*root,Relidsrelids)
1975+
{
1976+
doublerowcount=1.0;
1977+
intrelid;
1978+
1979+
relid=-1;
1980+
while ((relid=bms_next_member(relids,relid)) >=0)
1981+
{
1982+
RelOptInfo*rel;
1983+
1984+
/* Paranoia: ignore bogus relid indexes */
1985+
if (relid >=root->simple_rel_array_size)
1986+
continue;
1987+
rel=root->simple_rel_array[relid];
1988+
if (rel==NULL)
1989+
continue;
1990+
Assert(rel->relid==relid);/* sanity check on array */
1991+
1992+
/* Relation could be proven empty, if so ignore */
1993+
if (IS_DUMMY_REL(rel))
1994+
continue;
1995+
1996+
/* Otherwise, rel's rows estimate should be valid by now */
1997+
Assert(rel->rows>0);
1998+
1999+
/* Accumulate product */
2000+
rowcount *=rel->rows;
2001+
}
2002+
returnrowcount;
2003+
}
2004+
19062005

19072006
/****************************************************************************
19082007
*---- ROUTINES TO CHECK QUERY CLAUSES ----

‎src/backend/optimizer/path/joinrels.c‎

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -624,7 +624,10 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
624624
/* we don't bother trying to make the remaining fields valid */
625625
sjinfo->lhs_strict= false;
626626
sjinfo->delay_upper_joins= false;
627-
sjinfo->join_quals=NIL;
627+
sjinfo->semi_can_btree= false;
628+
sjinfo->semi_can_hash= false;
629+
sjinfo->semi_operators=NIL;
630+
sjinfo->semi_rhs_exprs=NIL;
628631
}
629632

630633
/*

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