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First cut at making useful selectivity estimates for range queries

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(ie, WHERE x > lowbound AND x < highbound).  It's not very bright yet
but it does something useful.  Also, rename intltsel/intgtsel to
scalarltsel/scalargtsel to reflect usage better.  Extend convert_to_scalar
to do something a little bit useful with string data types.  Still need
to make it do something with date/time datatypes, but I'll wait for
Thomas's datetime unification dust to settle first.  Eventually the
routine ought not have any type-specific knowledge at all; it ought to
be calling a type-dependent routine found via a pg_type column; but
that's a task for another day.
This commit is contained in:
Tom Lane
2000-01-24 07:16:52 +00:00
parent 8bcac56086
commit 0dbffa704a
9 changed files with 529 additions and 248 deletions
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245
src/backend/optimizer/path/clausesel.c
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@@ -7,7 +7,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.28 2000/01/23 02:06:58 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.29 2000/01/24 07:16:46 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -23,6 +23,23 @@
#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
****************************************************************************/
@@ -55,29 +72,237 @@ restrictlist_selectivity(Query *root,
* 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;
List *clause;
Selectivity s1 = 1.0;
RangeQueryClause *rqlist = NULL;
List *clist;
/* Use the product of the selectivities of the subclauses.
* XXX this is too optimistic, since the subclauses
* are very likely not independent...
/*
* 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(clause, clauses)
foreach(clist, clauses)
{
Selectivity s2 = clause_selectivity(root,
(Node *) lfirst(clause),
varRelid);
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;
if (s2 > 0.0)
{
/* All our hard work has paid off! */
s1 *= s2;
}
else
{
/* One or both is probably a default estimate,
* so punt and just merge them in generically.
*/
s1 *= rqlist->hibound * rqlist->lobound;
}
}
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.