Fix oversight in previous error-reporting patch; mustn't pfree path string

[PostgreSQL.git] / src / backend / optimizer / prep / prepqual.c

/*-------------------------------------------------------------------------
 *
 * prepqual.c
 *        Routines for preprocessing qualification expressions
 *
 *
 * The parser regards AND and OR as purely binary operators, so a qual like
 *              (A = 1) OR (A = 2) OR (A = 3) ...
 * will produce a nested parsetree
 *              (OR (A = 1) (OR (A = 2) (OR (A = 3) ...)))
 * In reality, the optimizer and executor regard AND and OR as N-argument
 * operators, so this tree can be flattened to
 *              (OR (A = 1) (A = 2) (A = 3) ...)
 *
 * Formerly, this module was responsible for doing the initial flattening,
 * but now we leave it to eval_const_expressions to do that since it has to
 * make a complete pass over the expression tree anyway.  Instead, we just
 * have to ensure that our manipulations preserve AND/OR flatness.
 * pull_ands() and pull_ors() are used to maintain flatness of the AND/OR
 * tree after local transformations that might introduce nested AND/ORs.
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "optimizer/clauses.h"
#include "optimizer/prep.h"
#include "utils/lsyscache.h"


static List *pull_ands(List *andlist);
static List *pull_ors(List *orlist);
static Expr *find_nots(Expr *qual);
static Expr *push_nots(Expr *qual);
static Expr *find_duplicate_ors(Expr *qual);
static Expr *process_duplicate_ors(List *orlist);


/*
 * canonicalize_qual
 *        Convert a qualification expression to the most useful form.
 *
 * The name of this routine is a holdover from a time when it would try to
 * force the expression into canonical AND-of-ORs or OR-of-ANDs form.
 * Eventually, we recognized that that had more theoretical purity than
 * actual usefulness, and so now the transformation doesn't involve any
 * notion of reaching a canonical form.
 *
 * NOTE: we assume the input has already been through eval_const_expressions
 * and therefore possesses AND/OR flatness.  Formerly this function included
 * its own flattening logic, but that requires a useless extra pass over the
 * tree.
 *
 * Returns the modified qualification.
 */
Expr *
canonicalize_qual(Expr *qual)
{
        Expr       *newqual;

        /* Quick exit for empty qual */
        if (qual == NULL)
                return NULL;

        /*
         * Push down NOTs.      We do this only in the top-level boolean expression,
         * without examining arguments of operators/functions. The main reason for
         * doing this is to expose as much top-level AND/OR structure as we can,
         * so there's no point in descending further.
         */
        newqual = find_nots(qual);

        /*
         * Pull up redundant subclauses in OR-of-AND trees.  Again, we do this
         * only within the top-level AND/OR structure.
         */
        newqual = find_duplicate_ors(newqual);

        return newqual;
}


/*
 * pull_ands
 *        Recursively flatten nested AND clauses into a single and-clause list.
 *
 * Input is the arglist of an AND clause.
 * Returns the rebuilt arglist (note original list structure is not touched).
 */
static List *
pull_ands(List *andlist)
{
        List       *out_list = NIL;
        ListCell   *arg;

        foreach(arg, andlist)
        {
                Node       *subexpr = (Node *) lfirst(arg);

                /*
                 * Note: we can destructively concat the subexpression's arglist
                 * because we know the recursive invocation of pull_ands will have
                 * built a new arglist not shared with any other expr. Otherwise we'd
                 * need a list_copy here.
                 */
                if (and_clause(subexpr))
                        out_list = list_concat(out_list,
                                                                   pull_ands(((BoolExpr *) subexpr)->args));
                else
                        out_list = lappend(out_list, subexpr);
        }
        return out_list;
}

/*
 * pull_ors
 *        Recursively flatten nested OR clauses into a single or-clause list.
 *
 * Input is the arglist of an OR clause.
 * Returns the rebuilt arglist (note original list structure is not touched).
 */
static List *
pull_ors(List *orlist)
{
        List       *out_list = NIL;
        ListCell   *arg;

        foreach(arg, orlist)
        {
                Node       *subexpr = (Node *) lfirst(arg);

                /*
                 * Note: we can destructively concat the subexpression's arglist
                 * because we know the recursive invocation of pull_ors will have
                 * built a new arglist not shared with any other expr. Otherwise we'd
                 * need a list_copy here.
                 */
                if (or_clause(subexpr))
                        out_list = list_concat(out_list,
                                                                   pull_ors(((BoolExpr *) subexpr)->args));
                else
                        out_list = lappend(out_list, subexpr);
        }
        return out_list;
}


/*
 * find_nots
 *        Traverse the qualification, looking for NOTs to take care of.
 *        For NOT clauses, apply push_nots() to try to push down the NOT.
 *        For AND and OR clause types, simply recurse.  Otherwise stop
 *        recursing (we do not worry about structure below the top AND/OR tree).
 *
 * Returns the modified qualification.  AND/OR flatness is preserved.
 */
static Expr *
find_nots(Expr *qual)
{
        if (and_clause((Node *) qual))
        {
                List       *t_list = NIL;
                ListCell   *temp;

                foreach(temp, ((BoolExpr *) qual)->args)
                        t_list = lappend(t_list, find_nots(lfirst(temp)));
                return make_andclause(pull_ands(t_list));
        }
        else if (or_clause((Node *) qual))
        {
                List       *t_list = NIL;
                ListCell   *temp;

                foreach(temp, ((BoolExpr *) qual)->args)
                        t_list = lappend(t_list, find_nots(lfirst(temp)));
                return make_orclause(pull_ors(t_list));
        }
        else if (not_clause((Node *) qual))
                return push_nots(get_notclausearg(qual));
        else
                return qual;
}

/*
 * push_nots
 *        Push down a NOT as far as possible.
 *
 * Input is an expression to be negated (e.g., the argument of a NOT clause).
 * Returns a new qual equivalent to the negation of the given qual.
 */
static Expr *
push_nots(Expr *qual)
{
        if (is_opclause(qual))
        {
                /*
                 * Negate an operator clause if possible: (NOT (< A B)) => (>= A B)
                 * Otherwise, retain the clause as it is (the NOT can't be pushed down
                 * any farther).
                 */
                OpExpr     *opexpr = (OpExpr *) qual;
                Oid                     negator = get_negator(opexpr->opno);

                if (negator)
                {
                        OpExpr     *newopexpr = makeNode(OpExpr);

                        newopexpr->opno = negator;
                        newopexpr->opfuncid = InvalidOid;
                        newopexpr->opresulttype = opexpr->opresulttype;
                        newopexpr->opretset = opexpr->opretset;
                        newopexpr->args = opexpr->args;
                        newopexpr->location = opexpr->location;
                        return (Expr *) newopexpr;
                }
                else
                        return make_notclause(qual);
        }
        else if (qual && IsA(qual, ScalarArrayOpExpr))
        {
                /*
                 * Negate a ScalarArrayOpExpr if there is a negator for its operator;
                 * for example x = ANY (list) becomes x <> ALL (list). Otherwise,
                 * retain the clause as it is (the NOT can't be pushed down any
                 * farther).
                 */
                ScalarArrayOpExpr *saopexpr = (ScalarArrayOpExpr *) qual;
                Oid                     negator = get_negator(saopexpr->opno);

                if (negator)
                {
                        ScalarArrayOpExpr *newopexpr = makeNode(ScalarArrayOpExpr);

                        newopexpr->opno = negator;
                        newopexpr->opfuncid = InvalidOid;
                        newopexpr->useOr = !saopexpr->useOr;
                        newopexpr->args = saopexpr->args;
                        newopexpr->location = saopexpr->location;
                        return (Expr *) newopexpr;
                }
                else
                        return make_notclause(qual);
        }
        else if (and_clause((Node *) qual))
        {
                /*--------------------
                 * Apply DeMorgan's Laws:
                 *              (NOT (AND A B)) => (OR (NOT A) (NOT B))
                 *              (NOT (OR A B))  => (AND (NOT A) (NOT B))
                 * i.e., swap AND for OR and negate all the subclauses.
                 *--------------------
                 */
                List       *t_list = NIL;
                ListCell   *temp;

                foreach(temp, ((BoolExpr *) qual)->args)
                        t_list = lappend(t_list, push_nots(lfirst(temp)));
                return make_orclause(pull_ors(t_list));
        }
        else if (or_clause((Node *) qual))
        {
                List       *t_list = NIL;
                ListCell   *temp;

                foreach(temp, ((BoolExpr *) qual)->args)
                        t_list = lappend(t_list, push_nots(lfirst(temp)));
                return make_andclause(pull_ands(t_list));
        }
        else if (not_clause((Node *) qual))
        {
                /*
                 * Another NOT cancels this NOT, so eliminate the NOT and stop
                 * negating this branch.  But search the subexpression for more NOTs
                 * to simplify.
                 */
                return find_nots(get_notclausearg(qual));
        }
        else
        {
                /*
                 * We don't know how to negate anything else, place a NOT at this
                 * level.  No point in recursing deeper, either.
                 */
                return make_notclause(qual);
        }
}


/*--------------------
 * The following code attempts to apply the inverse OR distributive law:
 *              ((A AND B) OR (A AND C))  =>  (A AND (B OR C))
 * That is, locate OR clauses in which every subclause contains an
 * identical term, and pull out the duplicated terms.
 *
 * This may seem like a fairly useless activity, but it turns out to be
 * applicable to many machine-generated queries, and there are also queries
 * in some of the TPC benchmarks that need it.  This was in fact almost the
 * sole useful side-effect of the old prepqual code that tried to force
 * the query into canonical AND-of-ORs form: the canonical equivalent of
 *              ((A AND B) OR (A AND C))
 * is
 *              ((A OR A) AND (A OR C) AND (B OR A) AND (B OR C))
 * which the code was able to simplify to
 *              (A AND (A OR C) AND (B OR A) AND (B OR C))
 * thus successfully extracting the common condition A --- but at the cost
 * of cluttering the qual with many redundant clauses.
 *--------------------
 */

/*
 * find_duplicate_ors
 *        Given a qualification tree with the NOTs pushed down, search for
 *        OR clauses to which the inverse OR distributive law might apply.
 *        Only the top-level AND/OR structure is searched.
 *
 * Returns the modified qualification.  AND/OR flatness is preserved.
 */
static Expr *
find_duplicate_ors(Expr *qual)
{
        if (or_clause((Node *) qual))
        {
                List       *orlist = NIL;
                ListCell   *temp;

                /* Recurse */
                foreach(temp, ((BoolExpr *) qual)->args)
                        orlist = lappend(orlist, find_duplicate_ors(lfirst(temp)));

                /*
                 * Don't need pull_ors() since this routine will never introduce an OR
                 * where there wasn't one before.
                 */
                return process_duplicate_ors(orlist);
        }
        else if (and_clause((Node *) qual))
        {
                List       *andlist = NIL;
                ListCell   *temp;

                /* Recurse */
                foreach(temp, ((BoolExpr *) qual)->args)
                        andlist = lappend(andlist, find_duplicate_ors(lfirst(temp)));
                /* Flatten any ANDs introduced just below here */
                andlist = pull_ands(andlist);
                /* The AND list can't get shorter, so result is always an AND */
                return make_andclause(andlist);
        }
        else
                return qual;
}

/*
 * process_duplicate_ors
 *        Given a list of exprs which are ORed together, try to apply
 *        the inverse OR distributive law.
 *
 * Returns the resulting expression (could be an AND clause, an OR
 * clause, or maybe even a single subexpression).
 */
static Expr *
process_duplicate_ors(List *orlist)
{
        List       *reference = NIL;
        int                     num_subclauses = 0;
        List       *winners;
        List       *neworlist;
        ListCell   *temp;

        if (orlist == NIL)
                return NULL;                    /* probably can't happen */
        if (list_length(orlist) == 1)           /* single-expression OR (can this
                                                                                 * happen?) */
                return linitial(orlist);

        /*
         * Choose the shortest AND clause as the reference list --- obviously, any
         * subclause not in this clause isn't in all the clauses. If we find a
         * clause that's not an AND, we can treat it as a one-element AND clause,
         * which necessarily wins as shortest.
         */
        foreach(temp, orlist)
        {
                Expr       *clause = (Expr *) lfirst(temp);

                if (and_clause((Node *) clause))
                {
                        List       *subclauses = ((BoolExpr *) clause)->args;
                        int                     nclauses = list_length(subclauses);

                        if (reference == NIL || nclauses < num_subclauses)
                        {
                                reference = subclauses;
                                num_subclauses = nclauses;
                        }
                }
                else
                {
                        reference = list_make1(clause);
                        break;
                }
        }

        /*
         * Just in case, eliminate any duplicates in the reference list.
         */
        reference = list_union(NIL, reference);

        /*
         * Check each element of the reference list to see if it's in all the OR
         * clauses.  Build a new list of winning clauses.
         */
        winners = NIL;
        foreach(temp, reference)
        {
                Expr       *refclause = (Expr *) lfirst(temp);
                bool            win = true;
                ListCell   *temp2;

                foreach(temp2, orlist)
                {
                        Expr       *clause = (Expr *) lfirst(temp2);

                        if (and_clause((Node *) clause))
                        {
                                if (!list_member(((BoolExpr *) clause)->args, refclause))
                                {
                                        win = false;
                                        break;
                                }
                        }
                        else
                        {
                                if (!equal(refclause, clause))
                                {
                                        win = false;
                                        break;
                                }
                        }
                }

                if (win)
                        winners = lappend(winners, refclause);
        }

        /*
         * If no winners, we can't transform the OR
         */
        if (winners == NIL)
                return make_orclause(orlist);

        /*
         * Generate new OR list consisting of the remaining sub-clauses.
         *
         * If any clause degenerates to empty, then we have a situation like (A
         * AND B) OR (A), which can be reduced to just A --- that is, the
         * additional conditions in other arms of the OR are irrelevant.
         *
         * Note that because we use list_difference, any multiple occurrences of a
         * winning clause in an AND sub-clause will be removed automatically.
         */
        neworlist = NIL;
        foreach(temp, orlist)
        {
                Expr       *clause = (Expr *) lfirst(temp);

                if (and_clause((Node *) clause))
                {
                        List       *subclauses = ((BoolExpr *) clause)->args;

                        subclauses = list_difference(subclauses, winners);
                        if (subclauses != NIL)
                        {
                                if (list_length(subclauses) == 1)
                                        neworlist = lappend(neworlist, linitial(subclauses));
                                else
                                        neworlist = lappend(neworlist, make_andclause(subclauses));
                        }
                        else
                        {
                                neworlist = NIL;        /* degenerate case, see above */
                                break;
                        }
                }
                else
                {
                        if (!list_member(winners, clause))
                                neworlist = lappend(neworlist, clause);
                        else
                        {
                                neworlist = NIL;        /* degenerate case, see above */
                                break;
                        }
                }
        }

        /*
         * Append reduced OR to the winners list, if it's not degenerate, handling
         * the special case of one element correctly (can that really happen?).
         * Also be careful to maintain AND/OR flatness in case we pulled up a
         * sub-sub-OR-clause.
         */
        if (neworlist != NIL)
        {
                if (list_length(neworlist) == 1)
                        winners = lappend(winners, linitial(neworlist));
                else
                        winners = lappend(winners, make_orclause(pull_ors(neworlist)));
        }

        /*
         * And return the constructed AND clause, again being wary of a single
         * element and AND/OR flatness.
         */
        if (list_length(winners) == 1)
                return (Expr *) linitial(winners);
        else
                return make_andclause(pull_ands(winners));
}