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Add support for deparsing semi-joins to contrib/postgres_fdw

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SEMI-JOIN is deparsed as the EXISTS subquery. It references outer and inner
relations, so it should be evaluated as the condition in the upper-level WHERE
clause. The signatures of deparseFromExprForRel() and deparseRangeTblRef() are
revised so that they can add conditions to the upper level.

PgFdwRelationInfo now has a hidden_subquery_rels field, referencing the relids
used in the inner parts of semi-join.  They can't be referred to from upper
relations and should be used internally for equivalence member searches.

The planner can create semi-join, which refers to inner rel vars in its target
list. However, we deparse semi-join as an exists() subquery. So we skip the
case when the target list references to inner rel of semi-join.

Author: Alexander Pyhalov
Reviewed-by: Ashutosh Bapat, Ian Lawrence Barwick, Yuuki Fujii, Tomas Vondra
Discussion: https://postgr.es/m/c9e2a757cf3ac2333714eaf83a9cc184@postgrespro.ru
This commit is contained in:
Alexander Korotkov
2023-12-05 22:53:12 +02:00
parent 278eb13c48
commit 824dbea3e4
5 changed files with 699 additions and 83 deletions
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94
contrib/postgres_fdw/postgres_fdw.c
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@ -779,6 +779,7 @@ postgresGetForeignRelSize(PlannerInfo *root,
fpinfo->make_outerrel_subquery = false;
fpinfo->make_innerrel_subquery = false;
fpinfo->lower_subquery_rels = NULL;
fpinfo->hidden_subquery_rels = NULL;
/* Set the relation index. */
fpinfo->relation_index = baserel->relid;
}
@ -5724,6 +5725,45 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
return commands;
}
/*
* Check if reltarget is safe enough to push down semi-join. Reltarget is not
* safe, if it contains references to inner rel relids, which do not belong to
* outer rel.
*/
static bool
semijoin_target_ok(PlannerInfo *root, RelOptInfo *joinrel, RelOptInfo *outerrel, RelOptInfo *innerrel)
{
List *vars;
ListCell *lc;
bool ok = true;
Assert(joinrel->reltarget);
vars = pull_var_clause((Node *) joinrel->reltarget->exprs, PVC_INCLUDE_PLACEHOLDERS);
foreach(lc, vars)
{
Var *var = (Var *) lfirst(lc);
if (!IsA(var, Var))
continue;
if (bms_is_member(var->varno, innerrel->relids) &&
!bms_is_member(var->varno, outerrel->relids))
{
/*
* The planner can create semi-join, which refers to inner rel
* vars in its target list. However, we deparse semi-join as an
* exists() subquery, so can't handle references to inner rel in
* the target list.
*/
ok = false;
break;
}
}
return ok;
}
/*
* Assess whether the join between inner and outer relations can be pushed down
* to the foreign server. As a side effect, save information we obtain in this
@ -5741,12 +5781,19 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
List *joinclauses;
/*
* We support pushing down INNER, LEFT, RIGHT and FULL OUTER joins.
* Constructing queries representing SEMI and ANTI joins is hard, hence
* not considered right now.
* We support pushing down INNER, LEFT, RIGHT, FULL OUTER and SEMI joins.
* Constructing queries representing ANTI joins is hard, hence not
* considered right now.
*/
if (jointype != JOIN_INNER && jointype != JOIN_LEFT &&
jointype != JOIN_RIGHT && jointype != JOIN_FULL)
jointype != JOIN_RIGHT && jointype != JOIN_FULL &&
jointype != JOIN_SEMI)
return false;
/*
* We can't push down semi-join if its reltarget is not safe
*/
if ((jointype == JOIN_SEMI) && !semijoin_target_ok(root, joinrel, outerrel, innerrel))
return false;
/*
@ -5858,6 +5905,8 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
Assert(bms_is_subset(fpinfo_i->lower_subquery_rels, innerrel->relids));
fpinfo->lower_subquery_rels = bms_union(fpinfo_o->lower_subquery_rels,
fpinfo_i->lower_subquery_rels);
fpinfo->hidden_subquery_rels = bms_union(fpinfo_o->hidden_subquery_rels,
fpinfo_i->hidden_subquery_rels);
/*
* Pull the other remote conditions from the joining relations into join
@ -5871,6 +5920,12 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
* the joinclauses, since they need to be evaluated while constructing the
* join.
*
* For SEMI-JOIN clauses from inner relation can not be added to
* remote_conds, but should be treated as join clauses (as they are
* deparsed to EXISTS subquery, where inner relation can be referred). A
* list of relation ids, which can't be referred to from higher levels, is
* preserved as a hidden_subquery_rels list.
*
* For a FULL OUTER JOIN, the other clauses from either relation can not
* be added to the joinclauses or remote_conds, since each relation acts
* as an outer relation for the other.
@ -5901,6 +5956,16 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
fpinfo_i->remote_conds);
break;
case JOIN_SEMI:
fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
fpinfo_i->remote_conds);
fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
fpinfo->remote_conds);
fpinfo->remote_conds = list_copy(fpinfo_o->remote_conds);
fpinfo->hidden_subquery_rels = bms_union(fpinfo->hidden_subquery_rels,
innerrel->relids);
break;
case JOIN_FULL:
/*
@ -5943,6 +6008,24 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
fpinfo->joinclauses = fpinfo->remote_conds;
fpinfo->remote_conds = NIL;
}
else if (jointype == JOIN_LEFT || jointype == JOIN_RIGHT || jointype == JOIN_FULL)
{
/*
* Conditions, generated from semi-joins, should be evaluated before
* LEFT/RIGHT/FULL join.
*/
if (!bms_is_empty(fpinfo_o->hidden_subquery_rels))
{
fpinfo->make_outerrel_subquery = true;
fpinfo->lower_subquery_rels = bms_add_members(fpinfo->lower_subquery_rels, outerrel->relids);
}
if (!bms_is_empty(fpinfo_i->hidden_subquery_rels))
{
fpinfo->make_innerrel_subquery = true;
fpinfo->lower_subquery_rels = bms_add_members(fpinfo->lower_subquery_rels, innerrel->relids);
}
}
/* Mark that this join can be pushed down safely */
fpinfo->pushdown_safe = true;
@ -7692,6 +7775,8 @@ find_em_for_rel(PlannerInfo *root, EquivalenceClass *ec, RelOptInfo *rel)
{
ListCell *lc;
PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) rel->fdw_private;
foreach(lc, ec->ec_members)
{
EquivalenceMember *em = (EquivalenceMember *) lfirst(lc);
@ -7702,6 +7787,7 @@ find_em_for_rel(PlannerInfo *root, EquivalenceClass *ec, RelOptInfo *rel)
*/
if (bms_is_subset(em->em_relids, rel->relids) &&
!bms_is_empty(em->em_relids) &&
bms_is_empty(bms_intersect(em->em_relids, fpinfo->hidden_subquery_rels)) &&
is_foreign_expr(root, rel, em->em_expr))
return em;
}