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postgres/src/backend/optimizer/path/clausesel.c

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/*-------------------------------------------------------------------------
*
* clausesel.c
* Routines to compute clause selectivities
*
* Portions Copyright (c) 1996-2000, PostgreSQL, Inc
* Portions Copyright (c) 1994, Regents of the University of California
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.36 2000/05/30 00:49:46 momjian Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "catalog/pg_operator.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/plancat.h"
#include "optimizer/restrictinfo.h"
#include "parser/parsetree.h"
#include "utils/fmgroids.h"
#include "utils/lsyscache.h"
/*
* Data structure for accumulating info about possible range-query
* clause pairs in clauselist_selectivity.
*/
typedef struct RangeQueryClause
{
struct RangeQueryClause *next; /* next in linked list */
Node *var; /* The common variable of the clauses */
bool have_lobound; /* found a low-bound clause yet? */
bool have_hibound; /* found a high-bound clause yet? */
Selectivity lobound; /* Selectivity of a var > something clause */
Selectivity hibound; /* Selectivity of a var < something clause */
} RangeQueryClause;
static void addRangeClause(RangeQueryClause **rqlist, Node *clause,
int flag, bool isLTsel, Selectivity s2);
/****************************************************************************
* ROUTINES TO COMPUTE SELECTIVITIES
****************************************************************************/
/*
* restrictlist_selectivity -
* Compute the selectivity of an implicitly-ANDed list of RestrictInfo
* clauses.
*
* This is the same as clauselist_selectivity except for the representation
* of the clause list.
*/
Selectivity
restrictlist_selectivity(Query *root,
List *restrictinfo_list,
int varRelid)
{
List *clauselist = get_actual_clauses(restrictinfo_list);
Selectivity result;
result = clauselist_selectivity(root, clauselist, varRelid);
freeList(clauselist);
return result;
}
/*
* clauselist_selectivity -
* Compute the selectivity of an implicitly-ANDed list of boolean
* expression clauses. The list can be empty, in which case 1.0
* must be returned.
*
* See clause_selectivity() for the meaning of the varRelid parameter.
*
* Our basic approach is to take the product of the selectivities of the
* subclauses. However, that's only right if the subclauses have independent
* probabilities, and in reality they are often NOT independent. So,
* we want to be smarter where we can.
* Currently, the only extra smarts we have is to recognize "range queries",
* such as "x > 34 AND x < 42". Clauses are recognized as possible range
* query components if they are restriction opclauses whose operators have
* scalarltsel() or scalargtsel() as their restriction selectivity estimator.
* We pair up clauses of this form that refer to the same variable. An
* unpairable clause of this kind is simply multiplied into the selectivity
* product in the normal way. But when we find a pair, we know that the
* selectivities represent the relative positions of the low and high bounds
* within the column's range, so instead of figuring the selectivity as
* hisel * losel, we can figure it as hisel + losel - 1. (To visualize this,
* see that hisel is the fraction of the range below the high bound, while
* losel is the fraction above the low bound; so hisel can be interpreted
* directly as a 0..1 value but we need to convert losel to 1-losel before
* interpreting it as a value. Then the available range is 1-losel to hisel.)
* If the calculation yields zero or negative, however, we chicken out and
* use the default interpretation; that probably means that one or both
* selectivities is a default estimate rather than an actual range value.
* Of course this is all very dependent on the behavior of
* scalarltsel/scalargtsel; perhaps some day we can generalize the approach.
*/
Selectivity
clauselist_selectivity(Query *root,
List *clauses,
int varRelid)
{
Selectivity s1 = 1.0;
RangeQueryClause *rqlist = NULL;
List *clist;
/*
* Initial scan over clauses. Anything that doesn't look like a
* potential rangequery clause gets multiplied into s1 and forgotten.
* Anything that does gets inserted into an rqlist entry.
*/
foreach(clist, clauses)
{
Node *clause = (Node *) lfirst(clist);
Selectivity s2;
/*
* See if it looks like a restriction clause with a constant. (If
* it's not a constant we can't really trust the selectivity!) NB:
* for consistency of results, this fragment of code had better
* match what clause_selectivity() would do.
*/
if (varRelid != 0 || NumRelids(clause) == 1)
{
int relidx;
AttrNumber attno;
Datum constval;
int flag;
get_relattval(clause, varRelid,
&relidx, &attno, &constval, &flag);
if (relidx != 0 && (flag & SEL_CONSTANT))
{
/* if get_relattval succeeded, it must be an opclause */
Oid opno = ((Oper *) ((Expr *) clause)->oper)->opno;
RegProcedure oprrest = get_oprrest(opno);
if (!oprrest)
s2 = (Selectivity) 0.5;
else
s2 = restriction_selectivity(oprrest, opno,
getrelid(relidx,
root->rtable),
attno,
constval, flag);
/*
* If we reach here, we have computed the same result that
* clause_selectivity would, so we can just use s2 if it's
* the wrong oprrest. But if it's the right oprrest, add
* the clause to rqlist for later processing.
*/
switch (oprrest)
{
case F_SCALARLTSEL:
addRangeClause(&rqlist, clause, flag, true, s2);
break;
case F_SCALARGTSEL:
addRangeClause(&rqlist, clause, flag, false, s2);
break;
default:
/* Just merge the selectivity in generically */
s1 = s1 * s2;
break;
}
continue; /* drop to loop bottom */
}
}
/* Not the right form, so treat it generically. */
s2 = clause_selectivity(root, clause, varRelid);
s1 = s1 * s2;
}
/*
* Now scan the rangequery pair list.
*/
while (rqlist != NULL)
{
RangeQueryClause *rqnext;
if (rqlist->have_lobound && rqlist->have_hibound)
{
/* Successfully matched a pair of range clauses */
Selectivity s2 = rqlist->hibound + rqlist->lobound - 1.0;
/*
* A zero or slightly negative s2 should be converted into a
* small positive value; we probably are dealing with a very
* tight range and got a bogus result due to roundoff errors.
* However, if s2 is very negative, then we probably have
* default selectivity estimates on one or both sides of the
* range. In that case, insert a not-so-wildly-optimistic
* default estimate.
*/
if (s2 <= 0.0)
{
if (s2 < -0.01)
{
/*
* No data available --- use a default estimate that
* is small, but not real small.
*/
s2 = 0.01;
}
else
{
/*
* It's just roundoff error; use a small positive
* value
*/
s2 = 1.0e-10;
}
}
/* Merge in the selectivity of the pair of clauses */
s1 *= s2;
}
else
{
/* Only found one of a pair, merge it in generically */
if (rqlist->have_lobound)
s1 *= rqlist->lobound;
else
s1 *= rqlist->hibound;
}
/* release storage and advance */
rqnext = rqlist->next;
pfree(rqlist);
rqlist = rqnext;
}
return s1;
}
/*
* addRangeClause --- add a new range clause for clauselist_selectivity
*
* Here is where we try to match up pairs of range-query clauses
*/
static void
addRangeClause(RangeQueryClause **rqlist, Node *clause,
int flag, bool isLTsel, Selectivity s2)
{
RangeQueryClause *rqelem;
Node *var;
bool is_lobound;
/* get_relattval sets flag&SEL_RIGHT if the var is on the LEFT. */
if (flag & SEL_RIGHT)
{
var = (Node *) get_leftop((Expr *) clause);
is_lobound = !isLTsel; /* x < something is high bound */
}
else
{
var = (Node *) get_rightop((Expr *) clause);
is_lobound = isLTsel; /* something < x is low bound */
}
for (rqelem = *rqlist; rqelem; rqelem = rqelem->next)
{
/*
* We use full equal() here because the "var" might be a function
* of one or more attributes of the same relation...
*/
if (!equal(var, rqelem->var))
continue;
/* Found the right group to put this clause in */
if (is_lobound)
{
if (!rqelem->have_lobound)
{
rqelem->have_lobound = true;
rqelem->lobound = s2;
}
else
{
/*
* We have found two similar clauses, such as x < y AND x
* < z. Keep only the more restrictive one.
*/
if (rqelem->lobound > s2)
rqelem->lobound = s2;
}
}
else
{
if (!rqelem->have_hibound)
{
rqelem->have_hibound = true;
rqelem->hibound = s2;
}
else
{
/*
* We have found two similar clauses, such as x > y AND x
* > z. Keep only the more restrictive one.
*/
if (rqelem->hibound > s2)
rqelem->hibound = s2;
}
}
return;
}
/* No matching var found, so make a new clause-pair data structure */
rqelem = (RangeQueryClause *) palloc(sizeof(RangeQueryClause));
rqelem->var = var;
if (is_lobound)
{
rqelem->have_lobound = true;
rqelem->have_hibound = false;
rqelem->lobound = s2;
}
else
{
rqelem->have_lobound = false;
rqelem->have_hibound = true;
rqelem->hibound = s2;
}
rqelem->next = *rqlist;
*rqlist = rqelem;
}
/*
* clause_selectivity -
* Compute the selectivity of a general boolean expression clause.
*
* varRelid is either 0 or a rangetable index.
*
* When varRelid is not 0, only variables belonging to that relation are
* considered in computing selectivity; other vars are treated as constants
* of unknown values. This is appropriate for estimating the selectivity of
* a join clause that is being used as a restriction clause in a scan of a
* nestloop join's inner relation --- varRelid should then be the ID of the
* inner relation.
*
* When varRelid is 0, all variables are treated as variables. This
* is appropriate for ordinary join clauses and restriction clauses.
*/
Selectivity
clause_selectivity(Query *root,
Node *clause,
int varRelid)
{
Selectivity s1 = 1.0; /* default for any unhandled clause type */
if (clause == NULL)
return s1;
if (IsA(clause, Var))
{
/*
* we have a bool Var. This is exactly equivalent to the clause:
* reln.attribute = 't' so we compute the selectivity as if that
* is what we have. The magic #define constants are a hack. I
* didn't want to have to do system cache look ups to find out all
* of that info.
*/
Index varno = ((Var *) clause)->varno;
if (varRelid == 0 || varRelid == (int) varno)
s1 = restriction_selectivity(F_EQSEL,
BooleanEqualOperator,
getrelid(varno, root->rtable),
((Var *) clause)->varattno,
Int8GetDatum(true),
SEL_CONSTANT | SEL_RIGHT);
/* an outer-relation bool var is taken as always true... */
}
else if (IsA(clause, Param))
{
/* XXX any way to do better? */
s1 = 1.0;
}
else if (IsA(clause, Const))
{
/* bool constant is pretty easy... */
s1 = ((bool) ((Const *) clause)->constvalue) ? 1.0 : 0.0;
}
else if (not_clause(clause))
{
/* inverse of the selectivity of the underlying clause */
s1 = 1.0 - clause_selectivity(root,
(Node *) get_notclausearg((Expr *) clause),
varRelid);
}
else if (and_clause(clause))
{
/* share code with clauselist_selectivity() */
s1 = clauselist_selectivity(root,
((Expr *) clause)->args,
varRelid);
}
else if (or_clause(clause))
{
/*
* Selectivities for an 'or' clause are computed as s1+s2 - s1*s2
* to account for the probable overlap of selected tuple sets. XXX
* is this too conservative?
*/
List *arg;
s1 = 0.0;
foreach(arg, ((Expr *) clause)->args)
{
Selectivity s2 = clause_selectivity(root,
(Node *) lfirst(arg),
varRelid);
s1 = s1 + s2 - s1 * s2;
}
}
else if (is_opclause(clause))
{
Oid opno = ((Oper *) ((Expr *) clause)->oper)->opno;
bool is_join_clause;
if (varRelid != 0)
{
/*
* If we are considering a nestloop join then all clauses are
* restriction clauses, since we are only interested in the
* one relation.
*/
is_join_clause = false;
}
else
{
/*
* Otherwise, it's a join if there's more than one relation
* used.
*/
is_join_clause = (NumRelids(clause) > 1);
}
if (is_join_clause)
{
/* Estimate selectivity for a join clause. */
RegProcedure oprjoin = get_oprjoin(opno);
/*
* if the oprjoin procedure is missing for whatever reason,
* use a selectivity of 0.5
*/
if (!oprjoin)
s1 = (Selectivity) 0.5;
else
{
int relid1,
relid2;
AttrNumber attno1,
attno2;
Oid reloid1,
reloid2;
get_rels_atts(clause, &relid1, &attno1, &relid2, &attno2);
reloid1 = relid1 ? getrelid(relid1, root->rtable) : InvalidOid;
reloid2 = relid2 ? getrelid(relid2, root->rtable) : InvalidOid;
s1 = join_selectivity(oprjoin, opno,
reloid1, attno1,
reloid2, attno2);
}
}
else
{
/* Estimate selectivity for a restriction clause. */
RegProcedure oprrest = get_oprrest(opno);
/*
* if the oprrest procedure is missing for whatever reason,
* use a selectivity of 0.5
*/
if (!oprrest)
s1 = (Selectivity) 0.5;
else
{
int relidx;
AttrNumber attno;
Datum constval;
int flag;
Oid reloid;
get_relattval(clause, varRelid,
&relidx, &attno, &constval, &flag);
reloid = relidx ? getrelid(relidx, root->rtable) : InvalidOid;
s1 = restriction_selectivity(oprrest, opno,
reloid, attno,
constval, flag);
}
}
}
else if (is_funcclause(clause))
{
/*
* This is not an operator, so we guess at the selectivity. THIS
* IS A HACK TO GET V4 OUT THE DOOR. FUNCS SHOULD BE ABLE TO HAVE
* SELECTIVITIES THEMSELVES. -- JMH 7/9/92
*/
s1 = (Selectivity) 0.3333333;
}
else if (is_subplan(clause))
{
/*
* Just for the moment! FIX ME! - vadim 02/04/98
*/
s1 = 1.0;
}
return s1;
}
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