prepqual.c

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/*-------------------------------------------------------------------------
 *
 * prepqual.c
 *    Routines for preprocessing qualification expressions
 *
 *
 * While the parser will produce flattened (N-argument) AND/OR trees from
 * simple sequences of AND'ed or OR'ed clauses, there might be an AND clause
 * directly underneath another AND, or OR underneath OR, if the input was
 * oddly parenthesized.  Also, rule expansion and subquery flattening could
 * produce such parsetrees.  The planner wants to flatten all such cases
 * to ensure consistent optimization behavior.
 *
 * 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) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/prep/prepqual.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/optimizer.h"
#include "optimizer/prep.h"
#include "utils/lsyscache.h"


static List *pull_ands(List *andlist);
static List *pull_ors(List *orlist);
static Expr *find_duplicate_ors(Expr *qual, bool is_check);
static Expr *process_duplicate_ors(List *orlist);


/*
 * negate_clause
 *    Negate a Boolean expression.
 *
 * Input is a clause to be negated (e.g., the argument of a NOT clause).
 * Returns a new clause equivalent to the negation of the given clause.
 *
 * Although this can be invoked on its own, it's mainly intended as a helper
 * for eval_const_expressions(), and that context drives several design
 * decisions.  In particular, if the input is already AND/OR flat, we must
 * preserve that property.  We also don't bother to recurse in situations
 * where we can assume that lower-level executions of eval_const_expressions
 * would already have simplified sub-clauses of the input.
 *
 * The difference between this and a simple make_notclause() is that this
 * tries to get rid of the NOT node by logical simplification.  It's clearly
 * always a win if the NOT node can be eliminated altogether.  However, our
 * use of DeMorgan's laws could result in having more NOT nodes rather than
 * fewer.  We do that unconditionally anyway, because in WHERE clauses it's
 * important to expose as much top-level AND/OR structure as possible.
 * Also, eliminating an intermediate NOT may allow us to flatten two levels
 * of AND or OR together that we couldn't have otherwise.  Finally, one of
 * the motivations for doing this is to ensure that logically equivalent
 * expressions will be seen as physically equal(), so we should always apply
 * the same transformations.
 */
Node *
negate_clause(Node *node)
{
    if (node == NULL)           /* should not happen */
        elog(ERROR, "can't negate an empty subexpression");
    switch (nodeTag(node))
    {
        case T_Const:
            {
                Const      *c = (Const *) node;

                /* NOT NULL is still NULL */
                if (c->constisnull)
                    return makeBoolConst(false, true);
                /* otherwise pretty easy */
                return makeBoolConst(!DatumGetBool(c->constvalue), false);
            }
            break;
        case T_OpExpr:
            {
                /*
                 * Negate operator if possible: (NOT (< A B)) => (>= A B)
                 */
                OpExpr     *opexpr = (OpExpr *) node;
                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->opcollid = opexpr->opcollid;
                    newopexpr->inputcollid = opexpr->inputcollid;
                    newopexpr->args = opexpr->args;
                    newopexpr->location = opexpr->location;
                    return (Node *) newopexpr;
                }
            }
            break;
        case T_ScalarArrayOpExpr:
            {
                /*
                 * Negate a ScalarArrayOpExpr if its operator has a negator;
                 * for example x = ANY (list) becomes x <> ALL (list)
                 */
                ScalarArrayOpExpr *saopexpr = (ScalarArrayOpExpr *) node;
                Oid         negator = get_negator(saopexpr->opno);

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

                    newopexpr->opno = negator;
                    newopexpr->opfuncid = InvalidOid;
                    newopexpr->useOr = !saopexpr->useOr;
                    newopexpr->inputcollid = saopexpr->inputcollid;
                    newopexpr->args = saopexpr->args;
                    newopexpr->location = saopexpr->location;
                    return (Node *) newopexpr;
                }
            }
            break;
        case T_BoolExpr:
            {
                BoolExpr   *expr = (BoolExpr *) node;

                switch (expr->boolop)
                {
                        /*--------------------
                         * 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 each subclause.
                         *
                         * If the input is already AND/OR flat and has no NOT
                         * directly above AND or OR, this transformation preserves
                         * those properties.  For example, if no direct child of
                         * the given AND clause is an AND or a NOT-above-OR, then
                         * the recursive calls of negate_clause() can't return any
                         * OR clauses.  So we needn't call pull_ors() before
                         * building a new OR clause.  Similarly for the OR case.
                         *--------------------
                         */
                    case AND_EXPR:
                        {
                            List       *nargs = NIL;
                            ListCell   *lc;

                            foreach(lc, expr->args)
                            {
                                nargs = lappend(nargs,
                                                negate_clause(lfirst(lc)));
                            }
                            return (Node *) make_orclause(nargs);
                        }
                        break;
                    case OR_EXPR:
                        {
                            List       *nargs = NIL;
                            ListCell   *lc;

                            foreach(lc, expr->args)
                            {
                                nargs = lappend(nargs,
                                                negate_clause(lfirst(lc)));
                            }
                            return (Node *) make_andclause(nargs);
                        }
                        break;
                    case NOT_EXPR:

                        /*
                         * NOT underneath NOT: they cancel.  We assume the
                         * input is already simplified, so no need to recurse.
                         */
                        return (Node *) linitial(expr->args);
                    default:
                        elog(ERROR, "unrecognized boolop: %d",
                             (int) expr->boolop);
                        break;
                }
            }
            break;
        case T_NullTest:
            {
                NullTest   *expr = (NullTest *) node;

                /*
                 * In the rowtype case, the two flavors of NullTest are *not*
                 * logical inverses, so we can't simplify.  But it does work
                 * for scalar datatypes.
                 */
                if (!expr->argisrow)
                {
                    NullTest   *newexpr = makeNode(NullTest);

                    newexpr->arg = expr->arg;
                    newexpr->nulltesttype = (expr->nulltesttype == IS_NULL ?
                                             IS_NOT_NULL : IS_NULL);
                    newexpr->argisrow = expr->argisrow;
                    newexpr->location = expr->location;
                    return (Node *) newexpr;
                }
            }
            break;
        case T_BooleanTest:
            {
                BooleanTest *expr = (BooleanTest *) node;
                BooleanTest *newexpr = makeNode(BooleanTest);

                newexpr->arg = expr->arg;
                switch (expr->booltesttype)
                {
                    case IS_TRUE:
                        newexpr->booltesttype = IS_NOT_TRUE;
                        break;
                    case IS_NOT_TRUE:
                        newexpr->booltesttype = IS_TRUE;
                        break;
                    case IS_FALSE:
                        newexpr->booltesttype = IS_NOT_FALSE;
                        break;
                    case IS_NOT_FALSE:
                        newexpr->booltesttype = IS_FALSE;
                        break;
                    case IS_UNKNOWN:
                        newexpr->booltesttype = IS_NOT_UNKNOWN;
                        break;
                    case IS_NOT_UNKNOWN:
                        newexpr->booltesttype = IS_UNKNOWN;
                        break;
                    default:
                        elog(ERROR, "unrecognized booltesttype: %d",
                             (int) expr->booltesttype);
                        break;
                }
                newexpr->location = expr->location;
                return (Node *) newexpr;
            }
            break;
        default:
            /* else fall through */
            break;
    }

    /*
     * Otherwise we don't know how to simplify this, so just tack on an
     * explicit NOT node.
     */
    return (Node *) make_notclause((Expr *) node);
}


/*
 * canonicalize_qual
 *    Convert a qualification expression to the most useful form.
 *
 * This is primarily intended to be used on top-level WHERE (or JOIN/ON)
 * clauses.  It can also be used on top-level CHECK constraints, for which
 * pass is_check = true.  DO NOT call it on any expression that is not known
 * to be one or the other, as it might apply inappropriate simplifications.
 *
 * 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, bool is_check)
{
    Expr       *newqual;

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

    /* This should not be invoked on quals in implicit-AND format */
    Assert(!IsA(qual, List));

    /*
     * Pull up redundant subclauses in OR-of-AND trees.  We do this only
     * within the top-level AND/OR structure; there's no point in looking
     * deeper.  Also remove any NULL constants in the top-level structure.
     */
    newqual = find_duplicate_ors(qual, is_check);

    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);

        if (is_andclause(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);

        if (is_orclause(subexpr))
            out_list = list_concat(out_list,
                                   pull_ors(((BoolExpr *) subexpr)->args));
        else
            out_list = lappend(out_list, subexpr);
    }
    return out_list;
}


/*--------------------
 * 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.
 *
 * While at it, we remove any NULL constants within the top-level AND/OR
 * structure, eg in a WHERE clause, "x OR NULL::boolean" is reduced to "x".
 * In general that would change the result, so eval_const_expressions can't
 * do it; but at top level of WHERE, we don't need to distinguish between
 * FALSE and NULL results, so it's valid to treat NULL::boolean the same
 * as FALSE and then simplify AND/OR accordingly.  Conversely, in a top-level
 * CHECK constraint, we may treat a NULL the same as TRUE.
 *
 * Returns the modified qualification.  AND/OR flatness is preserved.
 */
static Expr *
find_duplicate_ors(Expr *qual, bool is_check)
{
    if (is_orclause(qual))
    {
        List       *orlist = NIL;
        ListCell   *temp;

        /* Recurse */
        foreach(temp, ((BoolExpr *) qual)->args)
        {
            Expr       *arg = (Expr *) lfirst(temp);

            arg = find_duplicate_ors(arg, is_check);

            /* Get rid of any constant inputs */
            if (arg && IsA(arg, Const))
            {
                Const      *carg = (Const *) arg;

                if (is_check)
                {
                    /* Within OR in CHECK, drop constant FALSE */
                    if (!carg->constisnull && !DatumGetBool(carg->constvalue))
                        continue;
                    /* Constant TRUE or NULL, so OR reduces to TRUE */
                    return (Expr *) makeBoolConst(true, false);
                }
                else
                {
                    /* Within OR in WHERE, drop constant FALSE or NULL */
                    if (carg->constisnull || !DatumGetBool(carg->constvalue))
                        continue;
                    /* Constant TRUE, so OR reduces to TRUE */
                    return arg;
                }
            }

            orlist = lappend(orlist, arg);
        }

        /* Flatten any ORs pulled up to just below here */
        orlist = pull_ors(orlist);

        /* Now we can look for duplicate ORs */
        return process_duplicate_ors(orlist);
    }
    else if (is_andclause(qual))
    {
        List       *andlist = NIL;
        ListCell   *temp;

        /* Recurse */
        foreach(temp, ((BoolExpr *) qual)->args)
        {
            Expr       *arg = (Expr *) lfirst(temp);

            arg = find_duplicate_ors(arg, is_check);

            /* Get rid of any constant inputs */
            if (arg && IsA(arg, Const))
            {
                Const      *carg = (Const *) arg;

                if (is_check)
                {
                    /* Within AND in CHECK, drop constant TRUE or NULL */
                    if (carg->constisnull || DatumGetBool(carg->constvalue))
                        continue;
                    /* Constant FALSE, so AND reduces to FALSE */
                    return arg;
                }
                else
                {
                    /* Within AND in WHERE, drop constant TRUE */
                    if (!carg->constisnull && DatumGetBool(carg->constvalue))
                        continue;
                    /* Constant FALSE or NULL, so AND reduces to FALSE */
                    return (Expr *) makeBoolConst(false, false);
                }
            }

            andlist = lappend(andlist, arg);
        }

        /* Flatten any ANDs introduced just below here */
        andlist = pull_ands(andlist);

        /* AND of no inputs reduces to TRUE */
        if (andlist == NIL)
            return (Expr *) makeBoolConst(true, false);

        /* Single-expression AND just reduces to that expression */
        if (list_length(andlist) == 1)
            return (Expr *) linitial(andlist);

        /* Else we still need an AND node */
        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;

    /* OR of no inputs reduces to FALSE */
    if (orlist == NIL)
        return (Expr *) makeBoolConst(false, false);

    /* Single-expression OR just reduces to that expression */
    if (list_length(orlist) == 1)
        return (Expr *) 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 (is_andclause(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 (is_andclause(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 (is_andclause(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));
}