tidpath.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * tidpath.c
 *    Routines to determine which TID conditions are usable for scanning
 *    a given relation, and create TidPaths accordingly.
 *
 * What we are looking for here is WHERE conditions of the form
 * "CTID = pseudoconstant", which can be implemented by just fetching
 * the tuple directly via heap_fetch().  We can also handle OR'd conditions
 * such as (CTID = const1) OR (CTID = const2), as well as ScalarArrayOpExpr
 * conditions of the form CTID = ANY(pseudoconstant_array).  In particular
 * this allows
 *      WHERE ctid IN (tid1, tid2, ...)
 *
 * As with indexscans, our definition of "pseudoconstant" is pretty liberal:
 * we allow anything that doesn't involve a volatile function or a Var of
 * the relation under consideration.  Vars belonging to other relations of
 * the query are allowed, giving rise to parameterized TID scans.
 *
 * We also support "WHERE CURRENT OF cursor" conditions (CurrentOfExpr),
 * which amount to "CTID = run-time-determined-TID".  These could in
 * theory be translated to a simple comparison of CTID to the result of
 * a function, but in practice it works better to keep the special node
 * representation all the way through to execution.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/optimizer/path/tidpath.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/sysattr.h"
#include "catalog/pg_operator.h"
#include "catalog/pg_type.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/optimizer.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.h"


/*
 * Does this Var represent the CTID column of the specified baserel?
 */
static inline bool
IsCTIDVar(Var *var, RelOptInfo *rel)
{
    /* The vartype check is strictly paranoia */
    if (var->varattno == SelfItemPointerAttributeNumber &&
        var->vartype == TIDOID &&
        var->varno == rel->relid &&
        var->varlevelsup == 0)
        return true;
    return false;
}

/*
 * Check to see if a RestrictInfo is of the form
 *      CTID = pseudoconstant
 * or
 *      pseudoconstant = CTID
 * where the CTID Var belongs to relation "rel", and nothing on the
 * other side of the clause does.
 */
static bool
IsTidEqualClause(RestrictInfo *rinfo, RelOptInfo *rel)
{
    OpExpr     *node;
    Node       *arg1,
               *arg2,
               *other;
    Relids      other_relids;

    /* Must be an OpExpr */
    if (!is_opclause(rinfo->clause))
        return false;
    node = (OpExpr *) rinfo->clause;

    /* Operator must be tideq */
    if (node->opno != TIDEqualOperator)
        return false;
    Assert(list_length(node->args) == 2);
    arg1 = linitial(node->args);
    arg2 = lsecond(node->args);

    /* Look for CTID as either argument */
    other = NULL;
    other_relids = NULL;
    if (arg1 && IsA(arg1, Var) &&
        IsCTIDVar((Var *) arg1, rel))
    {
        other = arg2;
        other_relids = rinfo->right_relids;
    }
    if (!other && arg2 && IsA(arg2, Var) &&
        IsCTIDVar((Var *) arg2, rel))
    {
        other = arg1;
        other_relids = rinfo->left_relids;
    }
    if (!other)
        return false;

    /* The other argument must be a pseudoconstant */
    if (bms_is_member(rel->relid, other_relids) ||
        contain_volatile_functions(other))
        return false;

    return true;                /* success */
}

/*
 * Check to see if a RestrictInfo is of the form
 *      CTID = ANY (pseudoconstant_array)
 * where the CTID Var belongs to relation "rel", and nothing on the
 * other side of the clause does.
 */
static bool
IsTidEqualAnyClause(RestrictInfo *rinfo, RelOptInfo *rel)
{
    ScalarArrayOpExpr *node;
    Node       *arg1,
               *arg2;

    /* Must be a ScalarArrayOpExpr */
    if (!(rinfo->clause && IsA(rinfo->clause, ScalarArrayOpExpr)))
        return false;
    node = (ScalarArrayOpExpr *) rinfo->clause;

    /* Operator must be tideq */
    if (node->opno != TIDEqualOperator)
        return false;
    if (!node->useOr)
        return false;
    Assert(list_length(node->args) == 2);
    arg1 = linitial(node->args);
    arg2 = lsecond(node->args);

    /* CTID must be first argument */
    if (arg1 && IsA(arg1, Var) &&
        IsCTIDVar((Var *) arg1, rel))
    {
        /* The other argument must be a pseudoconstant */
        if (bms_is_member(rel->relid, pull_varnos(arg2)) ||
            contain_volatile_functions(arg2))
            return false;

        return true;            /* success */
    }

    return false;
}

/*
 * Check to see if a RestrictInfo is a CurrentOfExpr referencing "rel".
 */
static bool
IsCurrentOfClause(RestrictInfo *rinfo, RelOptInfo *rel)
{
    CurrentOfExpr *node;

    /* Must be a CurrentOfExpr */
    if (!(rinfo->clause && IsA(rinfo->clause, CurrentOfExpr)))
        return false;
    node = (CurrentOfExpr *) rinfo->clause;

    /* If it references this rel, we're good */
    if (node->cvarno == rel->relid)
        return true;

    return false;
}

/*
 * Extract a set of CTID conditions from the given RestrictInfo
 *
 * Returns a List of CTID qual RestrictInfos for the specified rel (with
 * implicit OR semantics across the list), or NIL if there are no usable
 * conditions.
 *
 * This function considers only base cases; AND/OR combination is handled
 * below.  Therefore the returned List never has more than one element.
 * (Using a List may seem a bit weird, but it simplifies the caller.)
 */
static List *
TidQualFromRestrictInfo(RestrictInfo *rinfo, RelOptInfo *rel)
{
    /*
     * We may ignore pseudoconstant clauses (they can't contain Vars, so could
     * not match anyway).
     */
    if (rinfo->pseudoconstant)
        return NIL;

    /*
     * If clause must wait till after some lower-security-level restriction
     * clause, reject it.
     */
    if (!restriction_is_securely_promotable(rinfo, rel))
        return NIL;

    /*
     * Check all base cases.  If we get a match, return the clause.
     */
    if (IsTidEqualClause(rinfo, rel) ||
        IsTidEqualAnyClause(rinfo, rel) ||
        IsCurrentOfClause(rinfo, rel))
        return list_make1(rinfo);

    return NIL;
}

/*
 * Extract a set of CTID conditions from implicit-AND List of RestrictInfos
 *
 * Returns a List of CTID qual RestrictInfos for the specified rel (with
 * implicit OR semantics across the list), or NIL if there are no usable
 * conditions.
 *
 * This function is just concerned with handling AND/OR recursion.
 */
static List *
TidQualFromRestrictInfoList(List *rlist, RelOptInfo *rel)
{
    List       *rlst = NIL;
    ListCell   *l;

    foreach(l, rlist)
    {
        RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);

        if (restriction_is_or_clause(rinfo))
        {
            ListCell   *j;

            /*
             * We must be able to extract a CTID condition from every
             * sub-clause of an OR, or we can't use it.
             */
            foreach(j, ((BoolExpr *) rinfo->orclause)->args)
            {
                Node       *orarg = (Node *) lfirst(j);
                List       *sublist;

                /* OR arguments should be ANDs or sub-RestrictInfos */
                if (is_andclause(orarg))
                {
                    List       *andargs = ((BoolExpr *) orarg)->args;

                    /* Recurse in case there are sub-ORs */
                    sublist = TidQualFromRestrictInfoList(andargs, rel);
                }
                else
                {
                    RestrictInfo *rinfo = castNode(RestrictInfo, orarg);

                    Assert(!restriction_is_or_clause(rinfo));
                    sublist = TidQualFromRestrictInfo(rinfo, rel);
                }

                /*
                 * If nothing found in this arm, we can't do anything with
                 * this OR clause.
                 */
                if (sublist == NIL)
                {
                    rlst = NIL; /* forget anything we had */
                    break;      /* out of loop over OR args */
                }

                /*
                 * OK, continue constructing implicitly-OR'ed result list.
                 */
                rlst = list_concat(rlst, sublist);
            }
        }
        else
        {
            /* Not an OR clause, so handle base cases */
            rlst = TidQualFromRestrictInfo(rinfo, rel);
        }

        /*
         * Stop as soon as we find any usable CTID condition.  In theory we
         * could get CTID equality conditions from different AND'ed clauses,
         * in which case we could try to pick the most efficient one.  In
         * practice, such usage seems very unlikely, so we don't bother; we
         * just exit as soon as we find the first candidate.
         */
        if (rlst)
            break;
    }

    return rlst;
}

/*
 * Given a list of join clauses involving our rel, create a parameterized
 * TidPath for each one that is a suitable TidEqual clause.
 *
 * In principle we could combine clauses that reference the same outer rels,
 * but it doesn't seem like such cases would arise often enough to be worth
 * troubling over.
 */
static void
BuildParameterizedTidPaths(PlannerInfo *root, RelOptInfo *rel, List *clauses)
{
    ListCell   *l;

    foreach(l, clauses)
    {
        RestrictInfo *rinfo = lfirst_node(RestrictInfo, l);
        List       *tidquals;
        Relids      required_outer;

        /*
         * Validate whether each clause is actually usable; we must check this
         * even when examining clauses generated from an EquivalenceClass,
         * since they might not satisfy the restriction on not having Vars of
         * our rel on the other side, or somebody might've built an operator
         * class that accepts type "tid" but has other operators in it.
         *
         * We currently consider only TidEqual join clauses.  In principle we
         * might find a suitable ScalarArrayOpExpr in the rel's joininfo list,
         * but it seems unlikely to be worth expending the cycles to check.
         * And we definitely won't find a CurrentOfExpr here.  Hence, we don't
         * use TidQualFromRestrictInfo; but this must match that function
         * otherwise.
         */
        if (rinfo->pseudoconstant ||
            !restriction_is_securely_promotable(rinfo, rel) ||
            !IsTidEqualClause(rinfo, rel))
            continue;

        /*
         * Check if clause can be moved to this rel; this is probably
         * redundant when considering EC-derived clauses, but we must check it
         * for "loose" join clauses.
         */
        if (!join_clause_is_movable_to(rinfo, rel))
            continue;

        /* OK, make list of clauses for this path */
        tidquals = list_make1(rinfo);

        /* Compute required outer rels for this path */
        required_outer = bms_union(rinfo->required_relids, rel->lateral_relids);
        required_outer = bms_del_member(required_outer, rel->relid);

        add_path(rel, (Path *) create_tidscan_path(root, rel, tidquals,
                                                   required_outer));
    }
}

/*
 * Test whether an EquivalenceClass member matches our rel's CTID Var.
 *
 * This is a callback for use by generate_implied_equalities_for_column.
 */
static bool
ec_member_matches_ctid(PlannerInfo *root, RelOptInfo *rel,
                       EquivalenceClass *ec, EquivalenceMember *em,
                       void *arg)
{
    if (em->em_expr && IsA(em->em_expr, Var) &&
        IsCTIDVar((Var *) em->em_expr, rel))
        return true;
    return false;
}

/*
 * create_tidscan_paths
 *    Create paths corresponding to direct TID scans of the given rel.
 *
 *    Candidate paths are added to the rel's pathlist (using add_path).
 */
void
create_tidscan_paths(PlannerInfo *root, RelOptInfo *rel)
{
    List       *tidquals;

    /*
     * If any suitable quals exist in the rel's baserestrict list, generate a
     * plain (unparameterized) TidPath with them.
     */
    tidquals = TidQualFromRestrictInfoList(rel->baserestrictinfo, rel);

    if (tidquals)
    {
        /*
         * This path uses no join clauses, but it could still have required
         * parameterization due to LATERAL refs in its tlist.
         */
        Relids      required_outer = rel->lateral_relids;

        add_path(rel, (Path *) create_tidscan_path(root, rel, tidquals,
                                                   required_outer));
    }

    /*
     * Try to generate parameterized TidPaths using equality clauses extracted
     * from EquivalenceClasses.  (This is important since simple "t1.ctid =
     * t2.ctid" clauses will turn into ECs.)
     */
    if (rel->has_eclass_joins)
    {
        List       *clauses;

        /* Generate clauses, skipping any that join to lateral_referencers */
        clauses = generate_implied_equalities_for_column(root,
                                                         rel,
                                                         ec_member_matches_ctid,
                                                         NULL,
                                                         rel->lateral_referencers);

        /* Generate a path for each usable join clause */
        BuildParameterizedTidPaths(root, rel, clauses);
    }

    /*
     * Also consider parameterized TidPaths using "loose" join quals.  Quals
     * of the form "t1.ctid = t2.ctid" would turn into these if they are outer
     * join quals, for example.
     */
    BuildParameterizedTidPaths(root, rel, rel->joininfo);
}