clausesel.c

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/*-------------------------------------------------------------------------
 *
 * clausesel.c
 *	  Routines to compute clause selectivities
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/clausesel.c,v 1.60 2003/08/04 02:40:00 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "catalog/pg_operator.h"
#include "catalog/pg_type.h"
#include "nodes/makefuncs.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"
#include "utils/selfuncs.h"


/* note that pg_type.h hardwires size of bool as 1 ... duplicate it */
#define MAKEBOOLCONST(val,isnull) \
	((Node *) makeConst(BOOLOID, 1, (Datum) (val), (isnull), true))


/*
 * 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,
			   bool varonleft, 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,
						 JoinType jointype)
{
	List	   *clauselist = get_actual_clauses(restrictinfo_list);
	Selectivity result;

	result = clauselist_selectivity(root, clauselist, varRelid, jointype);
	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 additional parameters.
 *
 * 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.
 * However, this calculation double-excludes nulls, so really we need
 * hisel + losel + null_frac - 1.)
 * If the calculation yields zero or negative, however, we chicken out and
 * use a default estimate; 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,
					   JoinType jointype)
{
	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 pseudoconstant
		 * on one side.  (Anything more complicated than that might not
		 * behave in the simple way we are expecting.)
		 *
		 * NB: for consistency of results, this fragment of code had better
		 * match what clause_selectivity() would do in the cases it
		 * handles.
		 */
		if (is_opclause(clause) &&
			(varRelid != 0 || NumRelids(clause) == 1))
		{
			OpExpr	   *expr = (OpExpr *) clause;

			if (length(expr->args) == 2)
			{
				bool		varonleft = true;

				if (is_pseudo_constant_clause(lsecond(expr->args)) ||
					(varonleft = false,
					 is_pseudo_constant_clause(lfirst(expr->args))))
				{
					Oid			opno = expr->opno;
					RegProcedure oprrest = get_oprrest(opno);

					s2 = restriction_selectivity(root, opno,
												 expr->args, varRelid);

					/*
					 * 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,
										   varonleft, true, s2);
							break;
						case F_SCALARGTSEL:
							addRangeClause(&rqlist, clause,
										   varonleft, 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, jointype);
		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;

			/* Adjust for double-exclusion of NULLs */
			s2 += nulltestsel(root, IS_NULL, rqlist->var, varRelid);

			/*
			 * 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 = 0.005;
				}
				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,
			   bool varonleft, bool isLTsel, Selectivity s2)
{
	RangeQueryClause *rqelem;
	Node	   *var;
	bool		is_lobound;

	if (varonleft)
	{
		var = get_leftop((Expr *) clause);
		is_lobound = !isLTsel;	/* x < something is high bound */
	}
	else
	{
		var = get_rightop((Expr *) clause);
		is_lobound = isLTsel;	/* something < x is low bound */
	}

	for (rqelem = *rqlist; rqelem; rqelem = rqelem->next)
	{
		/*