tidpath.c

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/*-------------------------------------------------------------------------
 *
 * 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, ...)
 *
 * 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.
 *
 * There is currently no special support for joins involving CTID; in
 * particular nothing corresponding to best_inner_indexscan().  Since it's
 * not very useful to store TIDs of one table in another table, there
 * doesn't seem to be enough use-case to justify adding a lot of code
 * for that.
 *
 *
 * Portions Copyright (c) 1996-2017, 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/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.h"


static bool IsTidEqualClause(OpExpr *node, int varno);
static bool IsTidEqualAnyClause(ScalarArrayOpExpr *node, int varno);
static List *TidQualFromExpr(Node *expr, int varno);
static List *TidQualFromBaseRestrictinfo(RelOptInfo *rel);


/*
 * Check to see if an opclause is of the form
 *      CTID = pseudoconstant
 * or
 *      pseudoconstant = CTID
 *
 * We check that the CTID Var belongs to relation "varno".  That is probably
 * redundant considering this is only applied to restriction clauses, but
 * let's be safe.
 */
static bool
IsTidEqualClause(OpExpr *node, int varno)
{
    Node       *arg1,
               *arg2,
               *other;
    Var        *var;

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

    /* Look for CTID as either argument */
    other = NULL;
    if (arg1 && IsA(arg1, Var))
    {
        var = (Var *) arg1;
        if (var->varattno == SelfItemPointerAttributeNumber &&
            var->vartype == TIDOID &&
            var->varno == varno &&
            var->varlevelsup == 0)
            other = arg2;
    }
    if (!other && arg2 && IsA(arg2, Var))
    {
        var = (Var *) arg2;
        if (var->varattno == SelfItemPointerAttributeNumber &&
            var->vartype == TIDOID &&
            var->varno == varno &&
            var->varlevelsup == 0)
            other = arg1;
    }
    if (!other)
        return false;
    if (exprType(other) != TIDOID)
        return false;           /* probably can't happen */

    /* The other argument must be a pseudoconstant */
    if (!is_pseudo_constant_clause(other))
        return false;

    return true;                /* success */
}

/*
 * Check to see if a clause is of the form
 *      CTID = ANY (pseudoconstant_array)
 */
static bool
IsTidEqualAnyClause(ScalarArrayOpExpr *node, int varno)
{
    Node       *arg1,
               *arg2;

    /* 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))
    {
        Var        *var = (Var *) arg1;

        if (var->varattno == SelfItemPointerAttributeNumber &&
            var->vartype == TIDOID &&
            var->varno == varno &&
            var->varlevelsup == 0)
        {
            /* The other argument must be a pseudoconstant */
            if (is_pseudo_constant_clause(arg2))
                return true;    /* success */
        }
    }

    return false;
}

/*
 *  Extract a set of CTID conditions from the given qual expression
 *
 *  Returns a List of CTID qual expressions (with implicit OR semantics
 *  across the list), or NIL if there are no usable conditions.
 *
 *  If the expression is an AND clause, we can use a CTID condition
 *  from any sub-clause.  If it is an OR clause, we must be able to
 *  extract a CTID condition from every sub-clause, or we can't use it.
 *
 *  In theory, in the AND case we could get CTID conditions from different
 *  sub-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.
 */
static List *
TidQualFromExpr(Node *expr, int varno)
{
    List       *rlst = NIL;
    ListCell   *l;

    if (is_opclause(expr))
    {
        /* base case: check for tideq opclause */
        if (IsTidEqualClause((OpExpr *) expr, varno))
            rlst = list_make1(expr);
    }
    else if (expr && IsA(expr, ScalarArrayOpExpr))
    {
        /* another base case: check for tid = ANY clause */
        if (IsTidEqualAnyClause((ScalarArrayOpExpr *) expr, varno))
            rlst = list_make1(expr);
    }
    else if (expr && IsA(expr, CurrentOfExpr))
    {
        /* another base case: check for CURRENT OF on this rel */
        if (((CurrentOfExpr *) expr)->cvarno == varno)
            rlst = list_make1(expr);
    }
    else if (and_clause(expr))
    {
        foreach(l, ((BoolExpr *) expr)->args)
        {
            rlst = TidQualFromExpr((Node *) lfirst(l), varno);
            if (rlst)
                break;
        }
    }
    else if (or_clause(expr))
    {
        foreach(l, ((BoolExpr *) expr)->args)
        {
            List       *frtn = TidQualFromExpr((Node *) lfirst(l), varno);

            if (frtn)
                rlst = list_concat(rlst, frtn);
            else
            {
                if (rlst)
                    list_free(rlst);
                rlst = NIL;
                break;
            }
        }
    }
    return rlst;
}

/*
 *  Extract a set of CTID conditions from the rel's baserestrictinfo list
 */
static List *
TidQualFromBaseRestrictinfo(RelOptInfo *rel)
{
    List       *rlst = NIL;
    ListCell   *l;

    foreach(l, rel->baserestrictinfo)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);

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

        rlst = TidQualFromExpr((Node *) rinfo->clause, rel->relid);
        if (rlst)
            break;
    }
    return rlst;
}

/*
 * 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)
{
    Relids      required_outer;
    List       *tidquals;

    /*
     * We don't support pushing join clauses into the quals of a tidscan, but
     * it could still have required parameterization due to LATERAL refs in
     * its tlist.
     */
    required_outer = rel->lateral_relids;

    tidquals = TidQualFromBaseRestrictinfo(rel);

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