planagg.c

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/*-------------------------------------------------------------------------
 *
 * planagg.c
 *    Special planning for aggregate queries.
 *
 * This module tries to replace MIN/MAX aggregate functions by subqueries
 * of the form
 *      (SELECT col FROM tab
 *       WHERE col IS NOT NULL AND existing-quals
 *       ORDER BY col ASC/DESC
 *       LIMIT 1)
 * Given a suitable index on tab.col, this can be much faster than the
 * generic scan-all-the-rows aggregation plan.  We can handle multiple
 * MIN/MAX aggregates by generating multiple subqueries, and their
 * orderings can be different.  However, if the query contains any
 * non-optimizable aggregates, there's no point since we'll have to
 * scan all the rows anyway.
 *
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/plan/planagg.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "catalog/pg_aggregate.h"
#include "catalog/pg_type.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/optimizer.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "optimizer/subselect.h"
#include "optimizer/tlist.h"
#include "parser/parse_clause.h"
#include "parser/parsetree.h"
#include "rewrite/rewriteManip.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"
 
static bool can_minmax_aggs(PlannerInfo *root, List **context);
static bool build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
                              Oid eqop, Oid sortop, bool nulls_first);
static void minmax_qp_callback(PlannerInfo *root, void *extra);
static Oid  fetch_agg_sort_op(Oid aggfnoid);
 
 
/*
 * preprocess_minmax_aggregates - preprocess MIN/MAX aggregates
 *
 * Check to see whether the query contains MIN/MAX aggregate functions that
 * might be optimizable via indexscans.  If it does, and all the aggregates
 * are potentially optimizable, then create a MinMaxAggPath and add it to
 * the (UPPERREL_GROUP_AGG, NULL) upperrel.
 *
 * This should be called by grouping_planner() just before it's ready to call
 * query_planner(), because we generate indexscan paths by cloning the
 * planner's state and invoking query_planner() on a modified version of
 * the query parsetree.  Thus, all preprocessing needed before query_planner()
 * must already be done.  This relies on the list of aggregates in
 * root->agginfos, so preprocess_aggrefs() must have been called already, too.
 */
void
preprocess_minmax_aggregates(PlannerInfo *root)
{
    Query      *parse = root->parse;
    FromExpr   *jtnode;
    RangeTblRef *rtr;
    RangeTblEntry *rte;
    List       *aggs_list;
    RelOptInfo *grouped_rel;
    ListCell   *lc;
 
    /* minmax_aggs list should be empty at this point */
    Assert(root->minmax_aggs == NIL);
 
    /* Nothing to do if query has no aggregates */
    if (!parse->hasAggs)
        return;
 
    Assert(!parse->setOperations);  /* shouldn't get here if a setop */
    Assert(parse->rowMarks == NIL); /* nor if FOR UPDATE */
 
    /*
     * Reject unoptimizable cases.
     *
     * We don't handle GROUP BY or windowing, because our current
     * implementations of grouping require looking at all the rows anyway, and
     * so there's not much point in optimizing MIN/MAX.
     */
    if (parse->groupClause || list_length(parse->groupingSets) > 1 ||
        parse->hasWindowFuncs)
        return;
 
    /*
     * Reject if query contains any CTEs; there's no way to build an indexscan
     * on one so we couldn't succeed here.  (If the CTEs are unreferenced,
     * that's not true, but it doesn't seem worth expending cycles to check.)
     */
    if (parse->cteList)
        return;
 
    /*
     * We also restrict the query to reference exactly one table, since join
     * conditions can't be handled reasonably.  (We could perhaps handle a
     * query containing cartesian-product joins, but it hardly seems worth the
     * trouble.)  However, the single table could be buried in several levels
     * of FromExpr due to subqueries.  Note the "single" table could be an
     * inheritance parent, too, including the case of a UNION ALL subquery
     * that's been flattened to an appendrel.
     */
    jtnode = parse->jointree;
    while (IsA(jtnode, FromExpr))
    {
        if (list_length(jtnode->fromlist) != 1)
            return;
        jtnode = linitial(jtnode->fromlist);
    }
    if (!IsA(jtnode, RangeTblRef))
        return;
    rtr = (RangeTblRef *) jtnode;
    rte = planner_rt_fetch(rtr->rtindex, root);
    if (rte->rtekind == RTE_RELATION)
         /* ordinary relation, ok */ ;
    else if (rte->rtekind == RTE_SUBQUERY && rte->inh)
         /* flattened UNION ALL subquery, ok */ ;
    else
        return;
 
    /*
     * Examine all the aggregates and verify all are MIN/MAX aggregates.  Stop
     * as soon as we find one that isn't.
     */
    aggs_list = NIL;
    if (!can_minmax_aggs(root, &aggs_list))
        return;
 
    /*
     * OK, there is at least the possibility of performing the optimization.
     * Build an access path for each aggregate.  If any of the aggregates
     * prove to be non-indexable, give up; there is no point in optimizing
     * just some of them.
     */
    foreach(lc, aggs_list)
    {
        MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);
        Oid         eqop;
        bool        reverse;
 
        /*
         * We'll need the equality operator that goes with the aggregate's
         * ordering operator.
         */
        eqop = get_equality_op_for_ordering_op(mminfo->aggsortop, &reverse);
        if (!OidIsValid(eqop))  /* shouldn't happen */
            elog(ERROR, "could not find equality operator for ordering operator %u",
                 mminfo->aggsortop);
 
        /*
         * We can use either an ordering that gives NULLS FIRST or one that
         * gives NULLS LAST; furthermore there's unlikely to be much
         * performance difference between them, so it doesn't seem worth
         * costing out both ways if we get a hit on the first one.  NULLS
         * FIRST is more likely to be available if the operator is a
         * reverse-sort operator, so try that first if reverse.
         */
        if (build_minmax_path(root, mminfo, eqop, mminfo->aggsortop, reverse))
            continue;
        if (build_minmax_path(root, mminfo, eqop, mminfo->aggsortop, !reverse))
            continue;
 
        /* No indexable path for this aggregate, so fail */
        return;
    }
 
    /*
     * OK, we can do the query this way.  Prepare to create a MinMaxAggPath
     * node.
     *
     * First, create an output Param node for each agg.  (If we end up not
     * using the MinMaxAggPath, we'll waste a PARAM_EXEC slot for each agg,
     * which is not worth worrying about.  We can't wait till create_plan time
     * to decide whether to make the Param, unfortunately.)
     */
    foreach(lc, aggs_list)
    {
        MinMaxAggInfo *mminfo = (MinMaxAggInfo *) lfirst(lc);
 
        mminfo->param =
            SS_make_initplan_output_param(root,
                                          exprType((Node *) mminfo->target),
                                          -1,
                                          exprCollation((Node *) mminfo->target));
    }
 
    /*
     * Create a MinMaxAggPath node with the appropriate estimated costs and
     * other needed data, and add it to the UPPERREL_GROUP_AGG upperrel, where
     * it will compete against the standard aggregate implementation.  (It
     * will likely always win, but we need not assume that here.)
     *
     * Note: grouping_planner won't have created this upperrel yet, but it's
     * fine for us to create it first.  We will not have inserted the correct
     * consider_parallel value in it, but MinMaxAggPath paths are currently
     * never parallel-safe anyway, so that doesn't matter.  Likewise, it
     * doesn't matter that we haven't filled FDW-related fields in the rel.
     * Also, because there are no rowmarks, we know that the processed_tlist
     * doesn't need to change anymore, so making the pathtarget now is safe.
     */
    grouped_rel = fetch_upper_rel(root, UPPERREL_GROUP_AGG, NULL);
    add_path(grouped_rel, (Path *)
             create_minmaxagg_path(root, grouped_rel,
                                   create_pathtarget(root,
                                                     root->processed_tlist),
                                   aggs_list,
                                   (List *) parse->havingQual));
}
 
/*
 * can_minmax_aggs
 *      Examine all the aggregates in the query, and check if they are
 *      all MIN/MAX aggregates.  If so, build a list of MinMaxAggInfo
 *      nodes for them.
 *
 * Returns false if a non-MIN/MAX aggregate is found, true otherwise.
 */
static bool
can_minmax_aggs(PlannerInfo *root, List **context)
{
    ListCell   *lc;
 
    /*
     * This function used to have to scan the query for itself, but now we can
     * just thumb through the AggInfo list made by preprocess_aggrefs.
     */
    foreach(lc, root->agginfos)
    {
        AggInfo    *agginfo = lfirst_node(AggInfo, lc);
        Aggref     *aggref = linitial_node(Aggref, agginfo->aggrefs);
        Oid         aggsortop;
        TargetEntry *curTarget;
        MinMaxAggInfo *mminfo;
 
        Assert(aggref->agglevelsup == 0);
        if (list_length(aggref->args) != 1)
            return false;       /* it couldn't be MIN/MAX */
 
        /*
         * ORDER BY is usually irrelevant for MIN/MAX, but it can change the
         * outcome if the aggsortop's operator class recognizes non-identical
         * values as equal.  For example, 4.0 and 4.00 are equal according to
         * numeric_ops, yet distinguishable.  If MIN() receives more than one
         * value equal to 4.0 and no value less than 4.0, it is unspecified
         * which of those equal values MIN() returns.  An ORDER BY expression
         * that differs for each of those equal values of the argument
         * expression makes the result predictable once again.  This is a
         * niche requirement, and we do not implement it with subquery paths.
         * In any case, this test lets us reject ordered-set aggregates
         * quickly.
         */
        if (aggref->aggorder != NIL)
            return false;
        /* note: we do not care if DISTINCT is mentioned ... */
 
        /*
         * We might implement the optimization when a FILTER clause is present
         * by adding the filter to the quals of the generated subquery.  For
         * now, just punt.
         */
        if (aggref->aggfilter != NULL)
            return false;
 
        aggsortop = fetch_agg_sort_op(aggref->aggfnoid);
        if (!OidIsValid(aggsortop))
            return false;       /* not a MIN/MAX aggregate */
 
        curTarget = (TargetEntry *) linitial(aggref->args);
 
        if (contain_mutable_functions((Node *) curTarget->expr))
            return false;       /* not potentially indexable */
 
        if (type_is_rowtype(exprType((Node *) curTarget->expr)))
            return false;       /* IS NOT NULL would have weird semantics */
 
        mminfo = makeNode(MinMaxAggInfo);
        mminfo->aggfnoid = aggref->aggfnoid;
        mminfo->aggsortop = aggsortop;
        mminfo->target = curTarget->expr;
        mminfo->subroot = NULL; /* don't compute path yet */
        mminfo->path = NULL;
        mminfo->pathcost = 0;
        mminfo->param = NULL;
 
        *context = lappend(*context, mminfo);
    }
    return true;
}
 
/*
 * build_minmax_path
 *      Given a MIN/MAX aggregate, try to build an indexscan Path it can be
 *      optimized with.
 *
 * If successful, stash the best path in *mminfo and return true.
 * Otherwise, return false.
 */
static bool
build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
                  Oid eqop, Oid sortop, bool nulls_first)
{
    PlannerInfo *subroot;
    Query      *parse;
    TargetEntry *tle;
    List       *tlist;
    NullTest   *ntest;
    SortGroupClause *sortcl;
    RelOptInfo *final_rel;
    Path       *sorted_path;
    Cost        path_cost;
    double      path_fraction;
 
    /*
     * We are going to construct what is effectively a sub-SELECT query, so
     * clone the current query level's state and adjust it to make it look
     * like a subquery.  Any outer references will now be one level higher
     * than before.  (This means that when we are done, there will be no Vars
     * of level 1, which is why the subquery can become an initplan.)
     */
    subroot = (PlannerInfo *) palloc(sizeof(PlannerInfo));
    memcpy(subroot, root, sizeof(PlannerInfo));
    subroot->query_level++;
    subroot->parent_root = root;
    /* reset subplan-related stuff */
    subroot->plan_params = NIL;
    subroot->outer_params = NULL;
    subroot->init_plans = NIL;
    subroot->agginfos = NIL;
    subroot->aggtransinfos = NIL;
 
    subroot->parse = parse = copyObject(root->parse);
    IncrementVarSublevelsUp((Node *) parse, 1, 1);
 
    /* append_rel_list might contain outer Vars? */
    subroot->append_rel_list = copyObject(root->append_rel_list);
    IncrementVarSublevelsUp((Node *) subroot->append_rel_list, 1, 1);
    /* There shouldn't be any OJ info to translate, as yet */
    Assert(subroot->join_info_list == NIL);
    /* and we haven't made equivalence classes, either */
    Assert(subroot->eq_classes == NIL);
    /* and we haven't created PlaceHolderInfos, either */
    Assert(subroot->placeholder_list == NIL);
 
    /*----------
     * Generate modified query of the form
     *      (SELECT col FROM tab
     *       WHERE col IS NOT NULL AND existing-quals
     *       ORDER BY col ASC/DESC
     *       LIMIT 1)
     *----------
     */
    /* single tlist entry that is the aggregate target */
    tle = makeTargetEntry(copyObject(mminfo->target),
                          (AttrNumber) 1,
                          pstrdup("agg_target"),
                          false);
    tlist = list_make1(tle);
    subroot->processed_tlist = parse->targetList = tlist;
 
    /* No HAVING, no DISTINCT, no aggregates anymore */
    parse->havingQual = NULL;
    subroot->hasHavingQual = false;
    parse->distinctClause = NIL;
    parse->hasDistinctOn = false;
    parse->hasAggs = false;
 
    /* Build "target IS NOT NULL" expression */
    ntest = makeNode(NullTest);
    ntest->nulltesttype = IS_NOT_NULL;
    ntest->arg = copyObject(mminfo->target);
    /* we checked it wasn't a rowtype in find_minmax_aggs_walker */
    ntest->argisrow = false;
    ntest->location = -1;
 
    /* User might have had that in WHERE already */
    if (!list_member((List *) parse->jointree->quals, ntest))
        parse->jointree->quals = (Node *)
            lcons(ntest, (List *) parse->jointree->quals);
 
    /* Build suitable ORDER BY clause */
    sortcl = makeNode(SortGroupClause);
    sortcl->tleSortGroupRef = assignSortGroupRef(tle, subroot->processed_tlist);
    sortcl->eqop = eqop;
    sortcl->sortop = sortop;
    sortcl->nulls_first = nulls_first;
    sortcl->hashable = false;   /* no need to make this accurate */
    parse->sortClause = list_make1(sortcl);
 
    /* set up expressions for LIMIT 1 */
    parse->limitOffset = NULL;
    parse->limitCount = (Node *) makeConst(INT8OID, -1, InvalidOid,
                                           sizeof(int64),
                                           Int64GetDatum(1), false,
                                           FLOAT8PASSBYVAL);
 
    /*
     * Generate the best paths for this query, telling query_planner that we
     * have LIMIT 1.
     */
    subroot->tuple_fraction = 1.0;
    subroot->limit_tuples = 1.0;
 
    final_rel = query_planner(subroot, minmax_qp_callback, NULL);
 
    /*
     * Since we didn't go through subquery_planner() to handle the subquery,
     * we have to do some of the same cleanup it would do, in particular cope
     * with params and initplans used within this subquery.  (This won't
     * matter if we end up not using the subplan.)
     */
    SS_identify_outer_params(subroot);
    SS_charge_for_initplans(subroot, final_rel);
 
    /*
     * Get the best presorted path, that being the one that's cheapest for
     * fetching just one row.  If there's no such path, fail.
     */
    if (final_rel->rows > 1.0)
        path_fraction = 1.0 / final_rel->rows;
    else
        path_fraction = 1.0;
 
    sorted_path =
        get_cheapest_fractional_path_for_pathkeys(final_rel->pathlist,
                                                  subroot->query_pathkeys,
                                                  NULL,
                                                  path_fraction);
    if (!sorted_path)
        return false;
 
    /*
     * The path might not return exactly what we want, so fix that.  (We
     * assume that this won't change any conclusions about which was the
     * cheapest path.)
     */
    sorted_path = apply_projection_to_path(subroot, final_rel, sorted_path,
                                           create_pathtarget(subroot,
                                                             subroot->processed_tlist));
 
    /*
     * Determine cost to get just the first row of the presorted path.
     *
     * Note: cost calculation here should match
     * compare_fractional_path_costs().
     */
    path_cost = sorted_path->startup_cost +
        path_fraction * (sorted_path->total_cost - sorted_path->startup_cost);
 
    /* Save state for further processing */
    mminfo->subroot = subroot;
    mminfo->path = sorted_path;
    mminfo->pathcost = path_cost;
 
    return true;
}
 
/*
 * Compute query_pathkeys and other pathkeys during query_planner()
 */
static void
minmax_qp_callback(PlannerInfo *root, void *extra)
{
    root->group_pathkeys = NIL;
    root->window_pathkeys = NIL;
    root->distinct_pathkeys = NIL;
 
    root->sort_pathkeys =
        make_pathkeys_for_sortclauses(root,
                                      root->parse->sortClause,
                                      root->parse->targetList);
 
    root->query_pathkeys = root->sort_pathkeys;
}
 
/*
 * Get the OID of the sort operator, if any, associated with an aggregate.
 * Returns InvalidOid if there is no such operator.
 */
static Oid
fetch_agg_sort_op(Oid aggfnoid)
{
    HeapTuple   aggTuple;
    Form_pg_aggregate aggform;
    Oid         aggsortop;
 
    /* fetch aggregate entry from pg_aggregate */
    aggTuple = SearchSysCache1(AGGFNOID, ObjectIdGetDatum(aggfnoid));
    if (!HeapTupleIsValid(aggTuple))
        return InvalidOid;
    aggform = (Form_pg_aggregate) GETSTRUCT(aggTuple);
    aggsortop = aggform->aggsortop;
    ReleaseSysCache(aggTuple);
 
    return aggsortop;
}