$PostgreSQL: pgsql/src/backend/optimizer/plan/README,v 1.3 2008/03/21 13:23:28 momjian Exp $

Subselects
==========

Vadim B. Mikheev


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Date: Fri, 13 Feb 1998 20:50:50 +0700
From: "Vadim B. Mikheev" <vadim@sable.krasnoyarsk.su>
Organization: ITTS (Krasnoyarsk)
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Subject: [HACKERS] Subselects are in CVS...
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This is some implementation notes and opened issues...

First, implementation uses new type of parameters - PARAM_EXEC - to deal
with correlation Vars. When query_planner() is called, it first tries to
replace all upper queries Var referenced in current query with Param of 
this type. Some global variables are used to keep mapping of Vars to 
Params and Params to Vars. 

After this, all current query' SubLinks are processed: for each SubLink 
found in query' qual union_planner() (old planner() function) will be 
called to plan corresponding subselect (union_planner() calls 
query_planner() for "simple" query and supports UNIONs). After subselect 
are planned, optimizer knows about is this correlated, un-correlated or 
_undirect_ correlated (references some grand-parent Vars but no parent 
ones: uncorrelated from the parent' point of view) query. 

For uncorrelated and undirect correlated subqueries of EXPRession or 
EXISTS type SubLinks will be replaced with "normal" clauses from
SubLink->Oper list (I changed this list to be list of EXPR nodes,
not just Oper ones). Right sides of these nodes are replaced with
PARAM_EXEC parameters. This is second use of new parameter type.
At run-time these parameters get value from result of subquery
evaluation (i.e. - from target list of subquery). Execution plan of
subquery itself becomes init plan of parent query. InitPlan knows
what parameters are to get values from subquery' results and will be
executed "on-demand" (for query select * from table where x > 0 and
y > (select max(a) from table_a) subquery will not be executed at all
if there are no tuples with x > 0 _and_ y is not used in index scan).

SubLinks for subqueries of all other types are transformed into
new type of Expr node - SUBPLAN_EXPR. Expr->args are just correlation
variables from _parent_ query. Expr->oper is new SubPlan node.

This node is used for InitPlan too. It keeps subquery range table,
indices of Params which are to get value from _parent_ query Vars
(i.e. - from Expr->args), indices of Params into which subquery'
results are to be substituted (this is for InitPlans), SubLink
and subquery' execution plan.

Plan node was changed to know about dependencies on Params from
parent queries and InitPlans, to keep list of changed Params
(from the above) and so be re-scanned if this list is not NULL.
Also, added list of InitPlans (actually, all of them for current
query are in topmost plan node now) and other SubPlans (from
plan->qual) - to initialize them and let them know about changed
Params (from the list of their "interests").

After all SubLinks are processed, query_planner() calls qual'
canonificator and does "normal" work. By using Params optimizer 
is mostly unchanged.

Well, Executor. To get subplans re-evaluated without ExecutorStart()
and ExecutorEnd() (without opening and closing relations and indices
and without many palloc() and pfree() - this is what SQL-funcs does
on each call) ExecReScan() now supports most of Plan types...

Explanation of EXPLAIN.

vac=> explain select * from tmp where x >= (select max(x2) from test2 
where y2 = y and exists (select * from tempx where tx = x));
NOTICE:  QUERY PLAN:

Seq Scan on tmp  (cost=40.03 size=101 width=8)
  SubPlan
  ^^^^^^^ subquery is in Seq Scan' qual, its plan is below
    ->  Aggregate  (cost=2.05 size=0 width=0)
          InitPlan
          ^^^^^^^^ EXISTS subsubquery is InitPlan of subquery
            ->  Seq Scan on tempx  (cost=4.33 size=1 width=4)
          ->  Result  (cost=2.05 size=0 width=0)
              ^^^^^^ EXISTS subsubquery was transformed into Param
                     and so we have Result node here
                ->  Index Scan on test2  (cost=2.05 size=1 width=4)


Opened issues.

1. No read permissions checking (easy, just not done yet).
2. readfuncs.c can't read subplan-s (easy, not critical, because of
   we currently nowhere use ascii representation of execution plans).
3. ExecReScan() doesn't support all plan types. At least support for
   MergeJoin has to be implemented.
4. Memory leaks in ExecReScan().
5. I need in advice: if subquery introduced with NOT IN doesn't return
   any tuples then qualification is failed, yes ?
6. Regression tests !!!!!!!!!!!!!!!!!!!!
   (Could we use data/queries from MySQL' crash.me ?
    Copyright-ed ? Could they give us rights ?)
7. Performance.
   - Should be good when subquery is transformed into InitPlan.
   - Something should be done for uncorrelated subqueries introduced
     with ANY/ALL - keep thinking. Currently, subplan will be re-scanned
     for each parent tuple - very slow...

Results of some test. TMP is table with x,y (int4-s), x in 0-9,
y = 100 - x, 1000 tuples (10 duplicates of each tuple). TEST2 is table 
with x2, y2 (int4-s), x2 in 1-99, y2 = 100 -x2, 10000 tuples (100 dups).

   Trying 

select * from tmp where x >= (select max(x2) from test2 where y2 = y);
   
   and

begin;
select y as ty, max(x2) as mx into table tsub from test2, tmp 
where y2 = y group by ty;
vacuum tsub;
select x, y from tmp, tsub where x >= mx and y = ty;
drop table tsub;
end;

   Without index on test2(y2):

SubSelect         -> 320 sec
Using temp table  -> 32 sec

   Having index

SubSelect         -> 17 sec (2M of memory)
Using temp table  -> 32 sec (12M of memory: -S 8192)

Vadim


