execIndexing.c

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/*-------------------------------------------------------------------------
 *
 * execIndexing.c
 *    routines for inserting index tuples and enforcing unique and
 *    exclusion constraints.
 *
 * ExecInsertIndexTuples() is the main entry point.  It's called after
 * inserting a tuple to the heap, and it inserts corresponding index tuples
 * into all indexes.  At the same time, it enforces any unique and
 * exclusion constraints:
 *
 * Unique Indexes
 * --------------
 *
 * Enforcing a unique constraint is straightforward.  When the index AM
 * inserts the tuple to the index, it also checks that there are no
 * conflicting tuples in the index already.  It does so atomically, so that
 * even if two backends try to insert the same key concurrently, only one
 * of them will succeed.  All the logic to ensure atomicity, and to wait
 * for in-progress transactions to finish, is handled by the index AM.
 *
 * If a unique constraint is deferred, we request the index AM to not
 * throw an error if a conflict is found.  Instead, we make note that there
 * was a conflict and return the list of indexes with conflicts to the
 * caller.  The caller must re-check them later, by calling index_insert()
 * with the UNIQUE_CHECK_EXISTING option.
 *
 * Exclusion Constraints
 * ---------------------
 *
 * Exclusion constraints are different from unique indexes in that when the
 * tuple is inserted to the index, the index AM does not check for
 * duplicate keys at the same time.  After the insertion, we perform a
 * separate scan on the index to check for conflicting tuples, and if one
 * is found, we throw an error and the transaction is aborted.  If the
 * conflicting tuple's inserter or deleter is in-progress, we wait for it
 * to finish first.
 *
 * There is a chance of deadlock, if two backends insert a tuple at the
 * same time, and then perform the scan to check for conflicts.  They will
 * find each other's tuple, and both try to wait for each other.  The
 * deadlock detector will detect that, and abort one of the transactions.
 * That's fairly harmless, as one of them was bound to abort with a
 * "duplicate key error" anyway, although you get a different error
 * message.
 *
 * If an exclusion constraint is deferred, we still perform the conflict
 * checking scan immediately after inserting the index tuple.  But instead
 * of throwing an error if a conflict is found, we return that information
 * to the caller.  The caller must re-check them later by calling
 * check_exclusion_constraint().
 *
 * Speculative insertion
 * ---------------------
 *
 * Speculative insertion is a two-phase mechanism used to implement
 * INSERT ... ON CONFLICT DO UPDATE/NOTHING.  The tuple is first inserted
 * to the heap and update the indexes as usual, but if a constraint is
 * violated, we can still back out the insertion without aborting the whole
 * transaction.  In an INSERT ... ON CONFLICT statement, if a conflict is
 * detected, the inserted tuple is backed out and the ON CONFLICT action is
 * executed instead.
 *
 * Insertion to a unique index works as usual: the index AM checks for
 * duplicate keys atomically with the insertion.  But instead of throwing
 * an error on a conflict, the speculatively inserted heap tuple is backed
 * out.
 *
 * Exclusion constraints are slightly more complicated.  As mentioned
 * earlier, there is a risk of deadlock when two backends insert the same
 * key concurrently.  That was not a problem for regular insertions, when
 * one of the transactions has to be aborted anyway, but with a speculative
 * insertion we cannot let a deadlock happen, because we only want to back
 * out the speculatively inserted tuple on conflict, not abort the whole
 * transaction.
 *
 * When a backend detects that the speculative insertion conflicts with
 * another in-progress tuple, it has two options:
 *
 * 1. back out the speculatively inserted tuple, then wait for the other
 *    transaction, and retry. Or,
 * 2. wait for the other transaction, with the speculatively inserted tuple
 *    still in place.
 *
 * If two backends insert at the same time, and both try to wait for each
 * other, they will deadlock.  So option 2 is not acceptable.  Option 1
 * avoids the deadlock, but it is prone to a livelock instead.  Both
 * transactions will wake up immediately as the other transaction backs
 * out.  Then they both retry, and conflict with each other again, lather,
 * rinse, repeat.
 *
 * To avoid the livelock, one of the backends must back out first, and then
 * wait, while the other one waits without backing out.  It doesn't matter
 * which one backs out, so we employ an arbitrary rule that the transaction
 * with the higher XID backs out.
 *
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/execIndexing.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/relscan.h"
#include "access/xact.h"
#include "catalog/index.h"
#include "executor/executor.h"
#include "nodes/nodeFuncs.h"
#include "storage/lmgr.h"
#include "utils/tqual.h"

/* waitMode argument to check_exclusion_or_unique_constraint() */
typedef enum
{
    CEOUC_WAIT,
    CEOUC_NOWAIT,
    CEOUC_LIVELOCK_PREVENTING_WAIT
} CEOUC_WAIT_MODE;

static bool check_exclusion_or_unique_constraint(Relation heap, Relation index,
                                     IndexInfo *indexInfo,
                                     ItemPointer tupleid,
                                     Datum *values, bool *isnull,
                                     EState *estate, bool newIndex,
                                     CEOUC_WAIT_MODE waitMode,
                                     bool errorOK,
                                     ItemPointer conflictTid);

static bool index_recheck_constraint(Relation index, Oid *constr_procs,
                         Datum *existing_values, bool *existing_isnull,
                         Datum *new_values);

/* ----------------------------------------------------------------
 *      ExecOpenIndices
 *
 *      Find the indices associated with a result relation, open them,
 *      and save information about them in the result ResultRelInfo.
 *
 *      At entry, caller has already opened and locked
 *      resultRelInfo->ri_RelationDesc.
 * ----------------------------------------------------------------
 */
void
ExecOpenIndices(ResultRelInfo *resultRelInfo, bool speculative)
{
    Relation    resultRelation = resultRelInfo->ri_RelationDesc;
    List       *indexoidlist;
    ListCell   *l;
    int         len,
                i;
    RelationPtr relationDescs;
    IndexInfo **indexInfoArray;

    resultRelInfo->ri_NumIndices = 0;

    /* fast path if no indexes */
    if (!RelationGetForm(resultRelation)->relhasindex)
        return;

    /*
     * Get cached list of index OIDs
     */
    indexoidlist = RelationGetIndexList(resultRelation);
    len = list_length(indexoidlist);
    if (len == 0)
        return;

    /*
     * allocate space for result arrays
     */
    relationDescs = (RelationPtr) palloc(len * sizeof(Relation));
    indexInfoArray = (IndexInfo **) palloc(len * sizeof(IndexInfo *));

    resultRelInfo->ri_NumIndices = len;
    resultRelInfo->ri_IndexRelationDescs = relationDescs;
    resultRelInfo->ri_IndexRelationInfo = indexInfoArray;

    /*
     * For each index, open the index relation and save pg_index info. We
     * acquire RowExclusiveLock, signifying we will update the index.
     *
     * Note: we do this even if the index is not IndexIsReady; it's not worth
     * the trouble to optimize for the case where it isn't.
     */
    i = 0;
    foreach(l, indexoidlist)
    {
        Oid         indexOid = lfirst_oid(l);
        Relation    indexDesc;
        IndexInfo  *ii;

        indexDesc = index_open(indexOid, RowExclusiveLock);

        /* extract index key information from the index's pg_index info */
        ii = BuildIndexInfo(indexDesc);

        /*
         * If the indexes are to be used for speculative insertion, add extra
         * information required by unique index entries.
         */
        if (speculative && ii->ii_Unique)
            BuildSpeculativeIndexInfo(indexDesc, ii);

        relationDescs[i] = indexDesc;
        indexInfoArray[i] = ii;
        i++;
    }

    list_free(indexoidlist);
}

/* ----------------------------------------------------------------
 *      ExecCloseIndices
 *
 *      Close the index relations stored in resultRelInfo
 * ----------------------------------------------------------------
 */
void
ExecCloseIndices(ResultRelInfo *resultRelInfo)
{
    int         i;
    int         numIndices;
    RelationPtr indexDescs;

    numIndices = resultRelInfo->ri_NumIndices;
    indexDescs = resultRelInfo->ri_IndexRelationDescs;

    for (i = 0; i < numIndices; i++)
    {
        if (indexDescs[i] == NULL)
            continue;           /* shouldn't happen? */

        /* Drop lock acquired by ExecOpenIndices */
        index_close(indexDescs[i], RowExclusiveLock);
    }

    /*
     * XXX should free indexInfo array here too?  Currently we assume that
     * such stuff will be cleaned up automatically in FreeExecutorState.
     */
}

/* ----------------------------------------------------------------
 *      ExecInsertIndexTuples
 *
 *      This routine takes care of inserting index tuples
 *      into all the relations indexing the result relation
 *      when a heap tuple is inserted into the result relation.
 *
 *      Unique and exclusion constraints are enforced at the same
 *      time.  This returns a list of index OIDs for any unique or
 *      exclusion constraints that are deferred and that had
 *      potential (unconfirmed) conflicts.  (if noDupErr == true,
 *      the same is done for non-deferred constraints, but report
 *      if conflict was speculative or deferred conflict to caller)
 *
 *      If 'arbiterIndexes' is nonempty, noDupErr applies only to
 *      those indexes.  NIL means noDupErr applies to all indexes.
 *
 *      CAUTION: this must not be called for a HOT update.
 *      We can't defend against that here for lack of info.
 *      Should we change the API to make it safer?
 * ----------------------------------------------------------------
 */
List *
ExecInsertIndexTuples(TupleTableSlot *slot,
                      ItemPointer tupleid,
                      EState *estate,
                      bool noDupErr,
                      bool *specConflict,
                      List *arbiterIndexes)
{
    List       *result = NIL;
    ResultRelInfo *resultRelInfo;
    int         i;
    int         numIndices;
    RelationPtr relationDescs;
    Relation    heapRelation;
    IndexInfo **indexInfoArray;
    ExprContext *econtext;
    Datum       values[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];

    /*
     * Get information from the result relation info structure.
     */
    resultRelInfo = estate->es_result_relation_info;
    numIndices = resultRelInfo->ri_NumIndices;
    relationDescs = resultRelInfo->ri_IndexRelationDescs;
    indexInfoArray = resultRelInfo->ri_IndexRelationInfo;
    heapRelation = resultRelInfo->ri_RelationDesc;

    /*
     * We will use the EState's per-tuple context for evaluating predicates
     * and index expressions (creating it if it's not already there).
     */
    econtext = GetPerTupleExprContext(estate);

    /* Arrange for econtext's scan tuple to be the tuple under test */
    econtext->ecxt_scantuple = slot;

    /*
     * for each index, form and insert the index tuple
     */
    for (i = 0; i < numIndices; i++)
    {
        Relation    indexRelation = relationDescs[i];
        IndexInfo  *indexInfo;
        bool        applyNoDupErr;
        IndexUniqueCheck checkUnique;
        bool        satisfiesConstraint;

        if (indexRelation == NULL)
            continue;

        indexInfo = indexInfoArray[i];

        /* If the index is marked as read-only, ignore it */
        if (!indexInfo->ii_ReadyForInserts)
            continue;

        /* Check for partial index */
        if (indexInfo->ii_Predicate != NIL)
        {
            ExprState  *predicate;

            /*
             * If predicate state not set up yet, create it (in the estate's
             * per-query context)
             */
            predicate = indexInfo->ii_PredicateState;
            if (predicate == NULL)
            {
                predicate = ExecPrepareQual(indexInfo->ii_Predicate, estate);
                indexInfo->ii_PredicateState = predicate;
            }

            /* Skip this index-update if the predicate isn't satisfied */
            if (!ExecQual(predicate, econtext))
                continue;
        }

        /*
         * FormIndexDatum fills in its values and isnull parameters with the
         * appropriate values for the column(s) of the index.
         */
        FormIndexDatum(indexInfo,
                       slot,
                       estate,
                       values,
                       isnull);

        /* Check whether to apply noDupErr to this index */
        applyNoDupErr = noDupErr &&
            (arbiterIndexes == NIL ||
             list_member_oid(arbiterIndexes,
                             indexRelation->rd_index->indexrelid));

        /*
         * The index AM does the actual insertion, plus uniqueness checking.
         *
         * For an immediate-mode unique index, we just tell the index AM to
         * throw error if not unique.
         *
         * For a deferrable unique index, we tell the index AM to just detect
         * possible non-uniqueness, and we add the index OID to the result
         * list if further checking is needed.
         *
         * For a speculative insertion (used by INSERT ... ON CONFLICT), do
         * the same as for a deferrable unique index.
         */
        if (!indexRelation->rd_index->indisunique)
            checkUnique = UNIQUE_CHECK_NO;
        else if (applyNoDupErr)
            checkUnique = UNIQUE_CHECK_PARTIAL;
        else if (indexRelation->rd_index->indimmediate)
            checkUnique = UNIQUE_CHECK_YES;
        else
            checkUnique = UNIQUE_CHECK_PARTIAL;

        satisfiesConstraint =
            index_insert(indexRelation, /* index relation */
                         values,    /* array of index Datums */
                         isnull,    /* null flags */
                         tupleid,   /* tid of heap tuple */
                         heapRelation,  /* heap relation */
                         checkUnique,   /* type of uniqueness check to do */
                         indexInfo);    /* index AM may need this */

        /*
         * If the index has an associated exclusion constraint, check that.
         * This is simpler than the process for uniqueness checks since we
         * always insert first and then check.  If the constraint is deferred,
         * we check now anyway, but don't throw error on violation or wait for
         * a conclusive outcome from a concurrent insertion; instead we'll
         * queue a recheck event.  Similarly, noDupErr callers (speculative
         * inserters) will recheck later, and wait for a conclusive outcome
         * then.
         *
         * An index for an exclusion constraint can't also be UNIQUE (not an
         * essential property, we just don't allow it in the grammar), so no
         * need to preserve the prior state of satisfiesConstraint.
         */
        if (indexInfo->ii_ExclusionOps != NULL)
        {
            bool        violationOK;
            CEOUC_WAIT_MODE waitMode;

            if (applyNoDupErr)
            {
                violationOK = true;
                waitMode = CEOUC_LIVELOCK_PREVENTING_WAIT;
            }
            else if (!indexRelation->rd_index->indimmediate)
            {
                violationOK = true;
                waitMode = CEOUC_NOWAIT;
            }
            else
            {
                violationOK = false;
                waitMode = CEOUC_WAIT;
            }

            satisfiesConstraint =
                check_exclusion_or_unique_constraint(heapRelation,
                                                     indexRelation, indexInfo,
                                                     tupleid, values, isnull,
                                                     estate, false,
                                                     waitMode, violationOK, NULL);
        }

        if ((checkUnique == UNIQUE_CHECK_PARTIAL ||
             indexInfo->ii_ExclusionOps != NULL) &&
            !satisfiesConstraint)
        {
            /*
             * The tuple potentially violates the uniqueness or exclusion
             * constraint, so make a note of the index so that we can re-check
             * it later.  Speculative inserters are told if there was a
             * speculative conflict, since that always requires a restart.
             */
            result = lappend_oid(result, RelationGetRelid(indexRelation));
            if (indexRelation->rd_index->indimmediate && specConflict)
                *specConflict = true;
        }
    }

    return result;
}

/* ----------------------------------------------------------------
 *      ExecCheckIndexConstraints
 *
 *      This routine checks if a tuple violates any unique or
 *      exclusion constraints.  Returns true if there is no conflict.
 *      Otherwise returns false, and the TID of the conflicting
 *      tuple is returned in *conflictTid.
 *
 *      If 'arbiterIndexes' is given, only those indexes are checked.
 *      NIL means all indexes.
 *
 *      Note that this doesn't lock the values in any way, so it's
 *      possible that a conflicting tuple is inserted immediately
 *      after this returns.  But this can be used for a pre-check
 *      before insertion.
 * ----------------------------------------------------------------
 */
bool
ExecCheckIndexConstraints(TupleTableSlot *slot,
                          EState *estate, ItemPointer conflictTid,
                          List *arbiterIndexes)
{
    ResultRelInfo *resultRelInfo;
    int         i;
    int         numIndices;
    RelationPtr relationDescs;
    Relation    heapRelation;
    IndexInfo **indexInfoArray;
    ExprContext *econtext;
    Datum       values[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    ItemPointerData invalidItemPtr;
    bool        checkedIndex = false;

    ItemPointerSetInvalid(conflictTid);
    ItemPointerSetInvalid(&invalidItemPtr);

    /*
     * Get information from the result relation info structure.
     */
    resultRelInfo = estate->es_result_relation_info;
    numIndices = resultRelInfo->ri_NumIndices;
    relationDescs = resultRelInfo->ri_IndexRelationDescs;
    indexInfoArray = resultRelInfo->ri_IndexRelationInfo;
    heapRelation = resultRelInfo->ri_RelationDesc;

    /*
     * We will use the EState's per-tuple context for evaluating predicates
     * and index expressions (creating it if it's not already there).
     */
    econtext = GetPerTupleExprContext(estate);

    /* Arrange for econtext's scan tuple to be the tuple under test */
    econtext->ecxt_scantuple = slot;

    /*
     * For each index, form index tuple and check if it satisfies the
     * constraint.
     */
    for (i = 0; i < numIndices; i++)
    {
        Relation    indexRelation = relationDescs[i];
        IndexInfo  *indexInfo;
        bool        satisfiesConstraint;

        if (indexRelation == NULL)
            continue;

        indexInfo = indexInfoArray[i];

        if (!indexInfo->ii_Unique && !indexInfo->ii_ExclusionOps)
            continue;

        /* If the index is marked as read-only, ignore it */
        if (!indexInfo->ii_ReadyForInserts)
            continue;

        /* When specific arbiter indexes requested, only examine them */
        if (arbiterIndexes != NIL &&
            !list_member_oid(arbiterIndexes,
                             indexRelation->rd_index->indexrelid))
            continue;

        if (!indexRelation->rd_index->indimmediate)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("ON CONFLICT does not support deferrable unique constraints/exclusion constraints as arbiters"),
                     errtableconstraint(heapRelation,
                                        RelationGetRelationName(indexRelation))));

        checkedIndex = true;

        /* Check for partial index */
        if (indexInfo->ii_Predicate != NIL)
        {
            ExprState  *predicate;

            /*
             * If predicate state not set up yet, create it (in the estate's
             * per-query context)
             */
            predicate = indexInfo->ii_PredicateState;
            if (predicate == NULL)
            {
                predicate = ExecPrepareQual(indexInfo->ii_Predicate, estate);
                indexInfo->ii_PredicateState = predicate;
            }

            /* Skip this index-update if the predicate isn't satisfied */
            if (!ExecQual(predicate, econtext))
                continue;
        }

        /*
         * FormIndexDatum fills in its values and isnull parameters with the
         * appropriate values for the column(s) of the index.
         */
        FormIndexDatum(indexInfo,
                       slot,
                       estate,
                       values,
                       isnull);

        satisfiesConstraint =
            check_exclusion_or_unique_constraint(heapRelation, indexRelation,
                                                 indexInfo, &invalidItemPtr,
                                                 values, isnull, estate, false,
                                                 CEOUC_WAIT, true,
                                                 conflictTid);
        if (!satisfiesConstraint)
            return false;
    }

    if (arbiterIndexes != NIL && !checkedIndex)
        elog(ERROR, "unexpected failure to find arbiter index");

    return true;
}

/*
 * Check for violation of an exclusion or unique constraint
 *
 * heap: the table containing the new tuple
 * index: the index supporting the constraint
 * indexInfo: info about the index, including the exclusion properties
 * tupleid: heap TID of the new tuple we have just inserted (invalid if we
 *      haven't inserted a new tuple yet)
 * values, isnull: the *index* column values computed for the new tuple
 * estate: an EState we can do evaluation in
 * newIndex: if true, we are trying to build a new index (this affects
 *      only the wording of error messages)
 * waitMode: whether to wait for concurrent inserters/deleters
 * violationOK: if true, don't throw error for violation
 * conflictTid: if not-NULL, the TID of the conflicting tuple is returned here
 *
 * Returns true if OK, false if actual or potential violation
 *
 * 'waitMode' determines what happens if a conflict is detected with a tuple
 * that was inserted or deleted by a transaction that's still running.
 * CEOUC_WAIT means that we wait for the transaction to commit, before
 * throwing an error or returning.  CEOUC_NOWAIT means that we report the
 * violation immediately; so the violation is only potential, and the caller
 * must recheck sometime later.  This behavior is convenient for deferred
 * exclusion checks; we need not bother queuing a deferred event if there is
 * definitely no conflict at insertion time.
 *
 * CEOUC_LIVELOCK_PREVENTING_WAIT is like CEOUC_NOWAIT, but we will sometimes
 * wait anyway, to prevent livelocking if two transactions try inserting at
 * the same time.  This is used with speculative insertions, for INSERT ON
 * CONFLICT statements. (See notes in file header)
 *
 * If violationOK is true, we just report the potential or actual violation to
 * the caller by returning 'false'.  Otherwise we throw a descriptive error
 * message here.  When violationOK is false, a false result is impossible.
 *
 * Note: The indexam is normally responsible for checking unique constraints,
 * so this normally only needs to be used for exclusion constraints.  But this
 * function is also called when doing a "pre-check" for conflicts on a unique
 * constraint, when doing speculative insertion.  Caller may use the returned
 * conflict TID to take further steps.
 */
static bool
check_exclusion_or_unique_constraint(Relation heap, Relation index,
                                     IndexInfo *indexInfo,
                                     ItemPointer tupleid,
                                     Datum *values, bool *isnull,
                                     EState *estate, bool newIndex,
                                     CEOUC_WAIT_MODE waitMode,
                                     bool violationOK,
                                     ItemPointer conflictTid)
{
    Oid        *constr_procs;
    uint16     *constr_strats;
    Oid        *index_collations = index->rd_indcollation;
    int         indnkeyatts = IndexRelationGetNumberOfKeyAttributes(index);
    IndexScanDesc index_scan;
    HeapTuple   tup;
    ScanKeyData scankeys[INDEX_MAX_KEYS];
    SnapshotData DirtySnapshot;
    int         i;
    bool        conflict;
    bool        found_self;
    ExprContext *econtext;
    TupleTableSlot *existing_slot;
    TupleTableSlot *save_scantuple;

    if (indexInfo->ii_ExclusionOps)
    {
        constr_procs = indexInfo->ii_ExclusionProcs;
        constr_strats = indexInfo->ii_ExclusionStrats;
    }
    else
    {
        constr_procs = indexInfo->ii_UniqueProcs;
        constr_strats = indexInfo->ii_UniqueStrats;
    }

    /*
     * If any of the input values are NULL, the constraint check is assumed to
     * pass (i.e., we assume the operators are strict).
     */
    for (i = 0; i < indnkeyatts; i++)
    {
        if (isnull[i])
            return true;
    }

    /*
     * Search the tuples that are in the index for any violations, including
     * tuples that aren't visible yet.
     */
    InitDirtySnapshot(DirtySnapshot);

    for (i = 0; i < indnkeyatts; i++)
    {
        ScanKeyEntryInitialize(&scankeys[i],
                               0,
                               i + 1,
                               constr_strats[i],
                               InvalidOid,
                               index_collations[i],
                               constr_procs[i],
                               values[i]);
    }

    /*
     * Need a TupleTableSlot to put existing tuples in.
     *
     * To use FormIndexDatum, we have to make the econtext's scantuple point
     * to this slot.  Be sure to save and restore caller's value for
     * scantuple.
     */
    existing_slot = MakeSingleTupleTableSlot(RelationGetDescr(heap));

    econtext = GetPerTupleExprContext(estate);
    save_scantuple = econtext->ecxt_scantuple;
    econtext->ecxt_scantuple = existing_slot;

    /*
     * May have to restart scan from this point if a potential conflict is
     * found.
     */
retry:
    conflict = false;
    found_self = false;
    index_scan = index_beginscan(heap, index, &DirtySnapshot, indnkeyatts, 0);
    index_rescan(index_scan, scankeys, indnkeyatts, NULL, 0);

    while ((tup = index_getnext(index_scan,
                                ForwardScanDirection)) != NULL)
    {
        TransactionId xwait;
        ItemPointerData ctid_wait;
        XLTW_Oper   reason_wait;
        Datum       existing_values[INDEX_MAX_KEYS];
        bool        existing_isnull[INDEX_MAX_KEYS];
        char       *error_new;
        char       *error_existing;

        /*
         * Ignore the entry for the tuple we're trying to check.
         */
        if (ItemPointerIsValid(tupleid) &&
            ItemPointerEquals(tupleid, &tup->t_self))
        {
            if (found_self)     /* should not happen */
                elog(ERROR, "found self tuple multiple times in index \"%s\"",
                     RelationGetRelationName(index));
            found_self = true;
            continue;
        }

        /*
         * Extract the index column values and isnull flags from the existing
         * tuple.
         */
        ExecStoreTuple(tup, existing_slot, InvalidBuffer, false);
        FormIndexDatum(indexInfo, existing_slot, estate,
                       existing_values, existing_isnull);

        /* If lossy indexscan, must recheck the condition */
        if (index_scan->xs_recheck)
        {
            if (!index_recheck_constraint(index,
                                          constr_procs,
                                          existing_values,
                                          existing_isnull,
                                          values))
                continue;       /* tuple doesn't actually match, so no
                                 * conflict */
        }

        /*
         * At this point we have either a conflict or a potential conflict.
         *
         * If an in-progress transaction is affecting the visibility of this
         * tuple, we need to wait for it to complete and then recheck (unless
         * the caller requested not to).  For simplicity we do rechecking by
         * just restarting the whole scan --- this case probably doesn't
         * happen often enough to be worth trying harder, and anyway we don't
         * want to hold any index internal locks while waiting.
         */
        xwait = TransactionIdIsValid(DirtySnapshot.xmin) ?
            DirtySnapshot.xmin : DirtySnapshot.xmax;

        if (TransactionIdIsValid(xwait) &&
            (waitMode == CEOUC_WAIT ||
             (waitMode == CEOUC_LIVELOCK_PREVENTING_WAIT &&
              DirtySnapshot.speculativeToken &&
              TransactionIdPrecedes(GetCurrentTransactionId(), xwait))))
        {
            ctid_wait = tup->t_data->t_ctid;
            reason_wait = indexInfo->ii_ExclusionOps ?
                XLTW_RecheckExclusionConstr : XLTW_InsertIndex;
            index_endscan(index_scan);
            if (DirtySnapshot.speculativeToken)
                SpeculativeInsertionWait(DirtySnapshot.xmin,
                                         DirtySnapshot.speculativeToken);
            else
                XactLockTableWait(xwait, heap, &ctid_wait, reason_wait);
            goto retry;
        }

        /*
         * We have a definite conflict (or a potential one, but the caller
         * didn't want to wait).  Return it to caller, or report it.
         */
        if (violationOK)
        {
            conflict = true;
            if (conflictTid)
                *conflictTid = tup->t_self;
            break;
        }

        error_new = BuildIndexValueDescription(index, values, isnull);
        error_existing = BuildIndexValueDescription(index, existing_values,
                                                    existing_isnull);
        if (newIndex)
            ereport(ERROR,
                    (errcode(ERRCODE_EXCLUSION_VIOLATION),
                     errmsg("could not create exclusion constraint \"%s\"",
                            RelationGetRelationName(index)),
                     error_new && error_existing ?
                     errdetail("Key %s conflicts with key %s.",
                               error_new, error_existing) :
                     errdetail("Key conflicts exist."),
                     errtableconstraint(heap,
                                        RelationGetRelationName(index))));
        else
            ereport(ERROR,
                    (errcode(ERRCODE_EXCLUSION_VIOLATION),
                     errmsg("conflicting key value violates exclusion constraint \"%s\"",
                            RelationGetRelationName(index)),
                     error_new && error_existing ?
                     errdetail("Key %s conflicts with existing key %s.",
                               error_new, error_existing) :
                     errdetail("Key conflicts with existing key."),
                     errtableconstraint(heap,
                                        RelationGetRelationName(index))));
    }

    index_endscan(index_scan);

    /*
     * Ordinarily, at this point the search should have found the originally
     * inserted tuple (if any), unless we exited the loop early because of
     * conflict.  However, it is possible to define exclusion constraints for
     * which that wouldn't be true --- for instance, if the operator is <>. So
     * we no longer complain if found_self is still false.
     */

    econtext->ecxt_scantuple = save_scantuple;

    ExecDropSingleTupleTableSlot(existing_slot);

    return !conflict;
}

/*
 * Check for violation of an exclusion constraint
 *
 * This is a dumbed down version of check_exclusion_or_unique_constraint
 * for external callers. They don't need all the special modes.
 */
void
check_exclusion_constraint(Relation heap, Relation index,
                           IndexInfo *indexInfo,
                           ItemPointer tupleid,
                           Datum *values, bool *isnull,
                           EState *estate, bool newIndex)
{
    (void) check_exclusion_or_unique_constraint(heap, index, indexInfo, tupleid,
                                                values, isnull,
                                                estate, newIndex,
                                                CEOUC_WAIT, false, NULL);
}

/*
 * Check existing tuple's index values to see if it really matches the
 * exclusion condition against the new_values.  Returns true if conflict.
 */
static bool
index_recheck_constraint(Relation index, Oid *constr_procs,
                         Datum *existing_values, bool *existing_isnull,
                         Datum *new_values)
{
    int         indnkeyatts = IndexRelationGetNumberOfKeyAttributes(index);
    int         i;

    for (i = 0; i < indnkeyatts; i++)
    {
        /* Assume the exclusion operators are strict */
        if (existing_isnull[i])
            return false;

        if (!DatumGetBool(OidFunctionCall2Coll(constr_procs[i],
                                               index->rd_indcollation[i],
                                               existing_values[i],
                                               new_values[i])))
            return false;
    }

    return true;
}