placeholder.c

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/*-------------------------------------------------------------------------
 *
 * placeholder.c
 *    PlaceHolderVar and PlaceHolderInfo manipulation routines
 *
 *
 * Portions Copyright (c) 1996-2013, 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/pathnode.h"
#include "optimizer/placeholder.h"
#include "optimizer/planmain.h"
#include "optimizer/var.h"
#include "utils/lsyscache.h"

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


/*
 * 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;
    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);
    phinfo->ph_eval_at = pull_varnos((Node *) phv);
    /* ph_eval_at may change later, see update_placeholder_eval_levels */
    phinfo->ph_needed = NULL;   /* initially it's unused */
    phinfo->ph_may_need = NULL;
    /* 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);

    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.
 */
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));
        (void) 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.
 *
 * The result is the set of base Relids contained in or below jtnode.
 * This is just an internal convenience, it's not used at the top level.
 */
static Relids
find_placeholders_recurse(PlannerInfo *root, Node *jtnode)
{
    Relids      jtrelids;

    if (jtnode == NULL)
        return NULL;
    if (IsA(jtnode, RangeTblRef))
    {
        int         varno = ((RangeTblRef *) jtnode)->rtindex;

        /* No quals to deal with, just return correct result */
        jtrelids = bms_make_singleton(varno);
    }
    else if (IsA(jtnode, FromExpr))
    {
        FromExpr   *f = (FromExpr *) jtnode;
        ListCell   *l;

        /*
         * First, recurse to handle child joins, and form their relid set.
         */
        jtrelids = NULL;
        foreach(l, f->fromlist)
        {
            Relids      sub_relids;

            sub_relids = find_placeholders_recurse(root, lfirst(l));
            jtrelids = bms_join(jtrelids, sub_relids);
        }

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

        /*
         * First, recurse to handle child joins, and form their relid set.
         */
        leftids = find_placeholders_recurse(root, j->larg);
        rightids = find_placeholders_recurse(root, j->rarg);
        jtrelids = bms_join(leftids, rightids);

        /* Process the qual clauses */
        mark_placeholders_in_expr(root, j->quals, jtrelids);
    }
    else
    {
        elog(ERROR, "unrecognized node type: %d",
             (int) nodeTag(jtnode));
        jtrelids = NULL;        /* keep compiler quiet */
    }
    return jtrelids;
}

/*
 * mark_placeholders_in_expr
 *      Find all PlaceHolderVars in the given expression, and mark them
 *      as possibly needed at the specified join level.
 *
 * relids is the syntactic join level to mark as the "maybe needed" level
 * for each PlaceHolderVar found in the expression.
 */
static void
mark_placeholders_in_expr(PlannerInfo *root, Node *expr, Relids relids)
{
    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_INCLUDE_PLACEHOLDERS);
    foreach(vl, vars)
    {
        PlaceHolderVar *phv = (PlaceHolderVar *) lfirst(vl);
        PlaceHolderInfo *phinfo;

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

        /* Create a PlaceHolderInfo entry if there's not one already */
        phinfo = find_placeholder_info(root, phv, true);

        /* Mark it, and recursively process any contained placeholders */
        mark_placeholder_maybe_needed(root, phinfo, relids);
    }
    list_free(vars);
}

/*
 * mark_placeholder_maybe_needed
 *      Mark a placeholder as possibly needed at the specified join level.
 *
 * relids is the syntactic join level to mark as the "maybe needed" level
 * for the placeholder.
 *
 * This is called during an initial scan of the query's targetlist and quals
 * before we begin deconstruct_jointree.  Once we begin deconstruct_jointree,
 * all active placeholders must be present in root->placeholder_list with
 * their correct ph_may_need values, because make_outerjoininfo and
 * update_placeholder_eval_levels require this info to be available while
 * we crawl up the join tree.
 */
void
mark_placeholder_maybe_needed(PlannerInfo *root, PlaceHolderInfo *phinfo,
                              Relids relids)
{
    Relids      est_eval_level;

    /* Mark the PHV as possibly needed at the given syntactic level */
    phinfo->ph_may_need = bms_add_members(phinfo->ph_may_need, relids);

    /*
     * This is a bit tricky: the PHV's contained expression may contain other,
     * lower-level PHVs.  We need to get those into the PlaceHolderInfo list,
     * but they aren't going to be needed where the outer PHV is referenced.
     * Rather, they'll be needed where the outer PHV is evaluated.  We can
     * estimate that conservatively as the syntactic location of the PHV's
     * expression, but not less than the level of any Vars it contains.
     * (Normally the Vars would come from below the syntactic location anyway,
     * but this might not be true if the PHV contains any LATERAL references.)
     */
    est_eval_level = bms_union(phinfo->ph_var->phrels, phinfo->ph_eval_at);

    /* Now recurse to take care of any such PHVs */
    mark_placeholders_in_expr(root, (Node *) phinfo->ph_var->phexpr,
                              est_eval_level);

    bms_free(est_eval_level);
}

/*
 * 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 subselect 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);

        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 join level
 * where the evaluation will be done.  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);
        Relids      eval_at = phinfo->ph_eval_at;

        /* No work unless it'll be evaluated above baserel level */
        if (bms_membership(eval_at) == BMS_MULTIPLE)
        {
            List       *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
                                               PVC_RECURSE_AGGREGATES,
                                               PVC_INCLUDE_PLACEHOLDERS);

            add_vars_to_targetlist(root, vars, 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;

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

            rel->reltargetlist = lappend(rel->reltargetlist,
                                         copyObject(phinfo->ph_var));
        }
    }
}

/*
 * add_placeholders_to_joinrel
 *      Add any required PlaceHolderVars to a join rel's targetlist.
 *
 * A join rel should emit a PlaceHolderVar if (a) the PHV is needed above
 * this join level and (b) the PHV can be computed at or below this level.
 * At this time we do not need to distinguish whether the PHV will be
 * computed here or copied up from below.
 */
void
add_placeholders_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel)
{
    Relids      relids = joinrel->relids;
    ListCell   *lc;

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

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