placeholder.c

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/*-------------------------------------------------------------------------
 *
 * placeholder.c
 *    PlaceHolderVar and PlaceHolderInfo manipulation routines
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/util/placeholder.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "nodes/nodeFuncs.h"
#include "optimizer/cost.h"
#include "optimizer/optimizer.h"
#include "optimizer/pathnode.h"
#include "optimizer/placeholder.h"
#include "optimizer/planmain.h"
#include "utils/lsyscache.h"

/* Local functions */
static void find_placeholders_recurse(PlannerInfo *root, Node *jtnode);
static void find_placeholders_in_expr(PlannerInfo *root, Node *expr);


/*
 * make_placeholder_expr
 *      Make a PlaceHolderVar for the given expression.
 *
 * phrels is the syntactic location (as a set of baserels) to attribute
 * to the expression.
 */
PlaceHolderVar *
make_placeholder_expr(PlannerInfo *root, Expr *expr, Relids phrels)
{
    PlaceHolderVar *phv = makeNode(PlaceHolderVar);

    phv->phexpr = expr;
    phv->phrels = phrels;
    phv->phid = ++(root->glob->lastPHId);
    phv->phlevelsup = 0;

    return phv;
}

/*
 * find_placeholder_info
 *      Fetch the PlaceHolderInfo for the given PHV
 *
 * If the PlaceHolderInfo doesn't exist yet, create it if create_new_ph is
 * true, else throw an error.
 *
 * This is separate from make_placeholder_expr because subquery pullup has
 * to make PlaceHolderVars for expressions that might not be used at all in
 * the upper query, or might not remain after const-expression simplification.
 * We build PlaceHolderInfos only for PHVs that are still present in the
 * simplified query passed to query_planner().
 *
 * Note: this should only be called after query_planner() has started.  Also,
 * create_new_ph must not be true after deconstruct_jointree begins, because
 * make_outerjoininfo assumes that we already know about all placeholders.
 */
PlaceHolderInfo *
find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
                      bool create_new_ph)
{
    PlaceHolderInfo *phinfo;
    Relids      rels_used;
    ListCell   *lc;

    /* if this ever isn't true, we'd need to be able to look in parent lists */
    Assert(phv->phlevelsup == 0);

    foreach(lc, root->placeholder_list)
    {
        phinfo = (PlaceHolderInfo *) lfirst(lc);
        if (phinfo->phid == phv->phid)
            return phinfo;
    }

    /* Not found, so create it */
    if (!create_new_ph)
        elog(ERROR, "too late to create a new PlaceHolderInfo");

    phinfo = makeNode(PlaceHolderInfo);

    phinfo->phid = phv->phid;
    phinfo->ph_var = copyObject(phv);

    /*
     * Any referenced rels that are outside the PHV's syntactic scope are
     * LATERAL references, which should be included in ph_lateral but not in
     * ph_eval_at.  If no referenced rels are within the syntactic scope,
     * force evaluation at the syntactic location.
     */
    rels_used = pull_varnos(root, (Node *) phv->phexpr);
    phinfo->ph_lateral = bms_difference(rels_used, phv->phrels);
    if (bms_is_empty(phinfo->ph_lateral))
        phinfo->ph_lateral = NULL;  /* make it exactly NULL if empty */
    phinfo->ph_eval_at = bms_int_members(rels_used, phv->phrels);
    /* If no contained vars, force evaluation at syntactic location */
    if (bms_is_empty(phinfo->ph_eval_at))
    {
        phinfo->ph_eval_at = bms_copy(phv->phrels);
        Assert(!bms_is_empty(phinfo->ph_eval_at));
    }
    /* ph_eval_at may change later, see update_placeholder_eval_levels */
    phinfo->ph_needed = NULL;   /* initially it's unused */
    /* for the moment, estimate width using just the datatype info */
    phinfo->ph_width = get_typavgwidth(exprType((Node *) phv->phexpr),
                                       exprTypmod((Node *) phv->phexpr));

    root->placeholder_list = lappend(root->placeholder_list, phinfo);

    /*
     * The PHV's contained expression may contain other, lower-level PHVs.  We
     * now know we need to get those into the PlaceHolderInfo list, too, so we
     * may as well do that immediately.
     */
    find_placeholders_in_expr(root, (Node *) phinfo->ph_var->phexpr);

    return phinfo;
}

/*
 * find_placeholders_in_jointree
 *      Search the jointree for PlaceHolderVars, and build PlaceHolderInfos
 *
 * We don't need to look at the targetlist because build_base_rel_tlists()
 * will already have made entries for any PHVs in the tlist.
 *
 * This is called before we begin deconstruct_jointree.  Once we begin
 * deconstruct_jointree, all active placeholders must be present in
 * root->placeholder_list, because make_outerjoininfo and
 * update_placeholder_eval_levels require this info to be available
 * while we crawl up the join tree.
 */
void
find_placeholders_in_jointree(PlannerInfo *root)
{
    /* We need do nothing if the query contains no PlaceHolderVars */
    if (root->glob->lastPHId != 0)
    {
        /* Start recursion at top of jointree */
        Assert(root->parse->jointree != NULL &&
               IsA(root->parse->jointree, FromExpr));
        find_placeholders_recurse(root, (Node *) root->parse->jointree);
    }
}

/*
 * find_placeholders_recurse
 *    One recursion level of find_placeholders_in_jointree.
 *
 * jtnode is the current jointree node to examine.
 */
static void
find_placeholders_recurse(PlannerInfo *root, Node *jtnode)
{
    if (jtnode == NULL)
        return;
    if (IsA(jtnode, RangeTblRef))
    {
        /* No quals to deal with here */
    }
    else if (IsA(jtnode, FromExpr))
    {
        FromExpr   *f = (FromExpr *) jtnode;
        ListCell   *l;

        /*
         * First, recurse to handle child joins.
         */
        foreach(l, f->fromlist)
        {
            find_placeholders_recurse(root, lfirst(l));
        }

        /*
         * Now process the top-level quals.
         */
        find_placeholders_in_expr(root, f->quals);
    }
    else if (IsA(jtnode, JoinExpr))
    {
        JoinExpr   *j = (JoinExpr *) jtnode;

        /*
         * First, recurse to handle child joins.
         */
        find_placeholders_recurse(root, j->larg);
        find_placeholders_recurse(root, j->rarg);

        /* Process the qual clauses */
        find_placeholders_in_expr(root, j->quals);
    }
    else
        elog(ERROR, "unrecognized node type: %d",
             (int) nodeTag(jtnode));
}

/*
 * find_placeholders_in_expr
 *      Find all PlaceHolderVars in the given expression, and create
 *      PlaceHolderInfo entries for them.
 */
static void
find_placeholders_in_expr(PlannerInfo *root, Node *expr)
{
    List       *vars;
    ListCell   *vl;

    /*
     * pull_var_clause does more than we need here, but it'll do and it's
     * convenient to use.
     */
    vars = pull_var_clause(expr,
                           PVC_RECURSE_AGGREGATES |
                           PVC_RECURSE_WINDOWFUNCS |
                           PVC_INCLUDE_PLACEHOLDERS);
    foreach(vl, vars)
    {
        PlaceHolderVar *phv = (PlaceHolderVar *) lfirst(vl);

        /* Ignore any plain Vars */
        if (!IsA(phv, PlaceHolderVar))
            continue;

        /* Create a PlaceHolderInfo entry if there's not one already */
        (void) find_placeholder_info(root, phv, true);
    }
    list_free(vars);
}

/*
 * update_placeholder_eval_levels
 *      Adjust the target evaluation levels for placeholders
 *
 * The initial eval_at level set by find_placeholder_info was the set of
 * rels used in the placeholder's expression (or the whole subselect below
 * the placeholder's syntactic location, if the expr is variable-free).
 * If the query contains any outer joins that can null any of those rels,
 * we must delay evaluation to above those joins.
 *
 * We repeat this operation each time we add another outer join to
 * root->join_info_list.  It's somewhat annoying to have to do that, but
 * since we don't have very much information on the placeholders' locations,
 * it's hard to avoid.  Each placeholder's eval_at level must be correct
 * by the time it starts to figure in outer-join delay decisions for higher
 * outer joins.
 *
 * In future we might want to put additional policy/heuristics here to
 * try to determine an optimal evaluation level.  The current rules will
 * result in evaluation at the lowest possible level.  However, pushing a
 * placeholder eval up the tree is likely to further constrain evaluation
 * order for outer joins, so it could easily be counterproductive; and we
 * don't have enough information at this point to make an intelligent choice.
 */
void
update_placeholder_eval_levels(PlannerInfo *root, SpecialJoinInfo *new_sjinfo)
{
    ListCell   *lc1;

    foreach(lc1, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc1);
        Relids      syn_level = phinfo->ph_var->phrels;
        Relids      eval_at;
        bool        found_some;
        ListCell   *lc2;

        /*
         * We don't need to do any work on this placeholder unless the
         * newly-added outer join is syntactically beneath its location.
         */
        if (!bms_is_subset(new_sjinfo->syn_lefthand, syn_level) ||
            !bms_is_subset(new_sjinfo->syn_righthand, syn_level))
            continue;

        /*
         * Check for delays due to lower outer joins.  This is the same logic
         * as in check_outerjoin_delay in initsplan.c, except that we don't
         * have anything to do with the delay_upper_joins flags; delay of
         * upper outer joins will be handled later, based on the eval_at
         * values we compute now.
         */
        eval_at = phinfo->ph_eval_at;

        do
        {
            found_some = false;
            foreach(lc2, root->join_info_list)
            {
                SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc2);

                /* disregard joins not within the PHV's sub-select */
                if (!bms_is_subset(sjinfo->syn_lefthand, syn_level) ||
                    !bms_is_subset(sjinfo->syn_righthand, syn_level))
                    continue;

                /* do we reference any nullable rels of this OJ? */
                if (bms_overlap(eval_at, sjinfo->min_righthand) ||
                    (sjinfo->jointype == JOIN_FULL &&
                     bms_overlap(eval_at, sjinfo->min_lefthand)))
                {
                    /* yes; have we included all its rels in eval_at? */
                    if (!bms_is_subset(sjinfo->min_lefthand, eval_at) ||
                        !bms_is_subset(sjinfo->min_righthand, eval_at))
                    {
                        /* no, so add them in */
                        eval_at = bms_add_members(eval_at,
                                                  sjinfo->min_lefthand);
                        eval_at = bms_add_members(eval_at,
                                                  sjinfo->min_righthand);
                        /* we'll need another iteration */
                        found_some = true;
                    }
                }
            }
        } while (found_some);

        /* Can't move the PHV's eval_at level to above its syntactic level */
        Assert(bms_is_subset(eval_at, syn_level));

        phinfo->ph_eval_at = eval_at;
    }
}

/*
 * fix_placeholder_input_needed_levels
 *      Adjust the "needed at" levels for placeholder inputs
 *
 * This is called after we've finished determining the eval_at levels for
 * all placeholders.  We need to make sure that all vars and placeholders
 * needed to evaluate each placeholder will be available at the scan or join
 * level where the evaluation will be done.  (It might seem that scan-level
 * evaluations aren't interesting, but that's not so: a LATERAL reference
 * within a placeholder's expression needs to cause the referenced var or
 * placeholder to be marked as needed in the scan where it's evaluated.)
 * Note that this loop can have side-effects on the ph_needed sets of other
 * PlaceHolderInfos; that's okay because we don't examine ph_needed here, so
 * there are no ordering issues to worry about.
 */
void
fix_placeholder_input_needed_levels(PlannerInfo *root)
{
    ListCell   *lc;

    foreach(lc, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
        List       *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
                                           PVC_RECURSE_AGGREGATES |
                                           PVC_RECURSE_WINDOWFUNCS |
                                           PVC_INCLUDE_PLACEHOLDERS);

        add_vars_to_targetlist(root, vars, phinfo->ph_eval_at, false);
        list_free(vars);
    }
}

/*
 * add_placeholders_to_base_rels
 *      Add any required PlaceHolderVars to base rels' targetlists.
 *
 * If any placeholder can be computed at a base rel and is needed above it,
 * add it to that rel's targetlist.  This might look like it could be merged
 * with fix_placeholder_input_needed_levels, but it must be separate because
 * join removal happens in between, and can change the ph_eval_at sets.  There
 * is essentially the same logic in add_placeholders_to_joinrel, but we can't
 * do that part until joinrels are formed.
 */
void
add_placeholders_to_base_rels(PlannerInfo *root)
{
    ListCell   *lc;

    foreach(lc, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
        Relids      eval_at = phinfo->ph_eval_at;
        int         varno;

        if (bms_get_singleton_member(eval_at, &varno) &&
            bms_nonempty_difference(phinfo->ph_needed, eval_at))
        {
            RelOptInfo *rel = find_base_rel(root, varno);

            rel->reltarget->exprs = lappend(rel->reltarget->exprs,
                                            copyObject(phinfo->ph_var));
            /* reltarget's cost and width fields will be updated later */
        }
    }
}

/*
 * add_placeholders_to_joinrel
 *      Add any required PlaceHolderVars to a join rel's targetlist;
 *      and if they contain lateral references, add those references to the
 *      joinrel's direct_lateral_relids.
 *
 * A join rel should emit a PlaceHolderVar if (a) the PHV can be computed
 * at or below this join level and (b) the PHV is needed above this level.
 * However, condition (a) is sufficient to add to direct_lateral_relids,
 * as explained below.
 */
void
add_placeholders_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel,
                            RelOptInfo *outer_rel, RelOptInfo *inner_rel)
{
    Relids      relids = joinrel->relids;
    ListCell   *lc;

    foreach(lc, root->placeholder_list)
    {
        PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);

        /* Is it computable here? */
        if (bms_is_subset(phinfo->ph_eval_at, relids))
        {
            /* Is it still needed above this joinrel? */
            if (bms_nonempty_difference(phinfo->ph_needed, relids))
            {
                /* Yup, add it to the output */
                joinrel->reltarget->exprs = lappend(joinrel->reltarget->exprs,
                                                    phinfo->ph_var);
                joinrel->reltarget->width += phinfo->ph_width;

                /*
                 * Charge the cost of evaluating the contained expression if
                 * the PHV can be computed here but not in either input.  This
                 * is a bit bogus because we make the decision based on the
                 * first pair of possible input relations considered for the
                 * joinrel.  With other pairs, it might be possible to compute
                 * the PHV in one input or the other, and then we'd be double
                 * charging the PHV's cost for some join paths.  For now, live
                 * with that; but we might want to improve it later by
                 * refiguring the reltarget costs for each pair of inputs.
                 */
                if (!bms_is_subset(phinfo->ph_eval_at, outer_rel->relids) &&
                    !bms_is_subset(phinfo->ph_eval_at, inner_rel->relids))
                {
                    QualCost    cost;

                    cost_qual_eval_node(&cost, (Node *) phinfo->ph_var->phexpr,
                                        root);
                    joinrel->reltarget->cost.startup += cost.startup;
                    joinrel->reltarget->cost.per_tuple += cost.per_tuple;
                }
            }

            /*
             * Also adjust joinrel's direct_lateral_relids to include the
             * PHV's source rel(s).  We must do this even if we're not
             * actually going to emit the PHV, otherwise join_is_legal() will
             * reject valid join orderings.  (In principle maybe we could
             * instead remove the joinrel's lateral_relids dependency; but
             * that's complicated to get right, and cases where we're not
             * going to emit the PHV are too rare to justify the work.)
             *
             * In principle we should only do this if the join doesn't yet
             * include the PHV's source rel(s).  But our caller
             * build_join_rel() will clean things up by removing the join's
             * own relids from its direct_lateral_relids, so we needn't
             * account for that here.
             */
            joinrel->direct_lateral_relids =
                bms_add_members(joinrel->direct_lateral_relids,
                                phinfo->ph_lateral);
        }
    }
}