matview.c

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/*-------------------------------------------------------------------------
 *
 * matview.c
 *    materialized view support
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/matview.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/tableam.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "catalog/catalog.h"
#include "catalog/indexing.h"
#include "catalog/namespace.h"
#include "catalog/pg_am.h"
#include "catalog/pg_opclass.h"
#include "catalog/pg_operator.h"
#include "commands/cluster.h"
#include "commands/matview.h"
#include "commands/tablecmds.h"
#include "commands/tablespace.h"
#include "executor/executor.h"
#include "executor/spi.h"
#include "miscadmin.h"
#include "parser/parse_relation.h"
#include "pgstat.h"
#include "rewrite/rewriteHandler.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"


typedef struct
{
    DestReceiver pub;           /* publicly-known function pointers */
    Oid         transientoid;   /* OID of new heap into which to store */
    /* These fields are filled by transientrel_startup: */
    Relation    transientrel;   /* relation to write to */
    CommandId   output_cid;     /* cmin to insert in output tuples */
    int         ti_options;     /* table_tuple_insert performance options */
    BulkInsertState bistate;    /* bulk insert state */
} DR_transientrel;

static int  matview_maintenance_depth = 0;

static void transientrel_startup(DestReceiver *self, int operation, TupleDesc typeinfo);
static bool transientrel_receive(TupleTableSlot *slot, DestReceiver *self);
static void transientrel_shutdown(DestReceiver *self);
static void transientrel_destroy(DestReceiver *self);
static uint64 refresh_matview_datafill(DestReceiver *dest, Query *query,
                                       const char *queryString);
static char *make_temptable_name_n(char *tempname, int n);
static void refresh_by_match_merge(Oid matviewOid, Oid tempOid, Oid relowner,
                                   int save_sec_context);
static void refresh_by_heap_swap(Oid matviewOid, Oid OIDNewHeap, char relpersistence);
static bool is_usable_unique_index(Relation indexRel);
static void OpenMatViewIncrementalMaintenance(void);
static void CloseMatViewIncrementalMaintenance(void);

/*
 * SetMatViewPopulatedState
 *      Mark a materialized view as populated, or not.
 *
 * NOTE: caller must be holding an appropriate lock on the relation.
 */
void
SetMatViewPopulatedState(Relation relation, bool newstate)
{
    Relation    pgrel;
    HeapTuple   tuple;

    Assert(relation->rd_rel->relkind == RELKIND_MATVIEW);

    /*
     * Update relation's pg_class entry.  Crucial side-effect: other backends
     * (and this one too!) are sent SI message to make them rebuild relcache
     * entries.
     */
    pgrel = table_open(RelationRelationId, RowExclusiveLock);
    tuple = SearchSysCacheCopy1(RELOID,
                                ObjectIdGetDatum(RelationGetRelid(relation)));
    if (!HeapTupleIsValid(tuple))
        elog(ERROR, "cache lookup failed for relation %u",
             RelationGetRelid(relation));

    ((Form_pg_class) GETSTRUCT(tuple))->relispopulated = newstate;

    CatalogTupleUpdate(pgrel, &tuple->t_self, tuple);

    heap_freetuple(tuple);
    table_close(pgrel, RowExclusiveLock);

    /*
     * Advance command counter to make the updated pg_class row locally
     * visible.
     */
    CommandCounterIncrement();
}

/*
 * ExecRefreshMatView -- execute a REFRESH MATERIALIZED VIEW command
 *
 * This refreshes the materialized view by creating a new table and swapping
 * the relfilenodes of the new table and the old materialized view, so the OID
 * of the original materialized view is preserved. Thus we do not lose GRANT
 * nor references to this materialized view.
 *
 * If WITH NO DATA was specified, this is effectively like a TRUNCATE;
 * otherwise it is like a TRUNCATE followed by an INSERT using the SELECT
 * statement associated with the materialized view.  The statement node's
 * skipData field shows whether the clause was used.
 *
 * Indexes are rebuilt too, via REINDEX. Since we are effectively bulk-loading
 * the new heap, it's better to create the indexes afterwards than to fill them
 * incrementally while we load.
 *
 * The matview's "populated" state is changed based on whether the contents
 * reflect the result set of the materialized view's query.
 */
ObjectAddress
ExecRefreshMatView(RefreshMatViewStmt *stmt, const char *queryString,
                   ParamListInfo params, QueryCompletion *qc)
{
    Oid         matviewOid;
    Relation    matviewRel;
    RewriteRule *rule;
    List       *actions;
    Query      *dataQuery;
    Oid         tableSpace;
    Oid         relowner;
    Oid         OIDNewHeap;
    DestReceiver *dest;
    uint64      processed = 0;
    bool        concurrent;
    LOCKMODE    lockmode;
    char        relpersistence;
    Oid         save_userid;
    int         save_sec_context;
    int         save_nestlevel;
    ObjectAddress address;

    /* Determine strength of lock needed. */
    concurrent = stmt->concurrent;
    lockmode = concurrent ? ExclusiveLock : AccessExclusiveLock;

    /*
     * Get a lock until end of transaction.
     */
    matviewOid = RangeVarGetRelidExtended(stmt->relation,
                                          lockmode, 0,
                                          RangeVarCallbackOwnsTable, NULL);
    matviewRel = table_open(matviewOid, NoLock);

    /* Make sure it is a materialized view. */
    if (matviewRel->rd_rel->relkind != RELKIND_MATVIEW)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("\"%s\" is not a materialized view",
                        RelationGetRelationName(matviewRel))));

    /* Check that CONCURRENTLY is not specified if not populated. */
    if (concurrent && !RelationIsPopulated(matviewRel))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("CONCURRENTLY cannot be used when the materialized view is not populated")));

    /* Check that conflicting options have not been specified. */
    if (concurrent && stmt->skipData)
        ereport(ERROR,
                (errcode(ERRCODE_SYNTAX_ERROR),
                 errmsg("CONCURRENTLY and WITH NO DATA options cannot be used together")));

    /*
     * Check that everything is correct for a refresh. Problems at this point
     * are internal errors, so elog is sufficient.
     */
    if (matviewRel->rd_rel->relhasrules == false ||
        matviewRel->rd_rules->numLocks < 1)
        elog(ERROR,
             "materialized view \"%s\" is missing rewrite information",
             RelationGetRelationName(matviewRel));

    if (matviewRel->rd_rules->numLocks > 1)
        elog(ERROR,
             "materialized view \"%s\" has too many rules",
             RelationGetRelationName(matviewRel));

    rule = matviewRel->rd_rules->rules[0];
    if (rule->event != CMD_SELECT || !(rule->isInstead))
        elog(ERROR,
             "the rule for materialized view \"%s\" is not a SELECT INSTEAD OF rule",
             RelationGetRelationName(matviewRel));

    actions = rule->actions;
    if (list_length(actions) != 1)
        elog(ERROR,
             "the rule for materialized view \"%s\" is not a single action",
             RelationGetRelationName(matviewRel));

    /*
     * Check that there is a unique index with no WHERE clause on one or more
     * columns of the materialized view if CONCURRENTLY is specified.
     */
    if (concurrent)
    {
        List       *indexoidlist = RelationGetIndexList(matviewRel);
        ListCell   *indexoidscan;
        bool        hasUniqueIndex = false;

        foreach(indexoidscan, indexoidlist)
        {
            Oid         indexoid = lfirst_oid(indexoidscan);
            Relation    indexRel;

            indexRel = index_open(indexoid, AccessShareLock);
            hasUniqueIndex = is_usable_unique_index(indexRel);
            index_close(indexRel, AccessShareLock);
            if (hasUniqueIndex)
                break;
        }

        list_free(indexoidlist);

        if (!hasUniqueIndex)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("cannot refresh materialized view \"%s\" concurrently",
                            quote_qualified_identifier(get_namespace_name(RelationGetNamespace(matviewRel)),
                                                       RelationGetRelationName(matviewRel))),
                     errhint("Create a unique index with no WHERE clause on one or more columns of the materialized view.")));
    }

    /*
     * The stored query was rewritten at the time of the MV definition, but
     * has not been scribbled on by the planner.
     */
    dataQuery = linitial_node(Query, actions);

    /*
     * Check for active uses of the relation in the current transaction, such
     * as open scans.
     *
     * NB: We count on this to protect us against problems with refreshing the
     * data using TABLE_INSERT_FROZEN.
     */
    CheckTableNotInUse(matviewRel, "REFRESH MATERIALIZED VIEW");

    /*
     * Tentatively mark the matview as populated or not (this will roll back
     * if we fail later).
     */
    SetMatViewPopulatedState(matviewRel, !stmt->skipData);

    relowner = matviewRel->rd_rel->relowner;

    /*
     * Switch to the owner's userid, so that any functions are run as that
     * user.  Also arrange to make GUC variable changes local to this command.
     * Don't lock it down too tight to create a temporary table just yet.  We
     * will switch modes when we are about to execute user code.
     */
    GetUserIdAndSecContext(&save_userid, &save_sec_context);
    SetUserIdAndSecContext(relowner,
                           save_sec_context | SECURITY_LOCAL_USERID_CHANGE);
    save_nestlevel = NewGUCNestLevel();

    /* Concurrent refresh builds new data in temp tablespace, and does diff. */
    if (concurrent)
    {
        tableSpace = GetDefaultTablespace(RELPERSISTENCE_TEMP, false);
        relpersistence = RELPERSISTENCE_TEMP;
    }
    else
    {
        tableSpace = matviewRel->rd_rel->reltablespace;
        relpersistence = matviewRel->rd_rel->relpersistence;
    }

    /*
     * Create the transient table that will receive the regenerated data. Lock
     * it against access by any other process until commit (by which time it
     * will be gone).
     */
    OIDNewHeap = make_new_heap(matviewOid, tableSpace, relpersistence,
                               ExclusiveLock);
    LockRelationOid(OIDNewHeap, AccessExclusiveLock);
    dest = CreateTransientRelDestReceiver(OIDNewHeap);

    /*
     * Now lock down security-restricted operations.
     */
    SetUserIdAndSecContext(relowner,
                           save_sec_context | SECURITY_RESTRICTED_OPERATION);

    /* Generate the data, if wanted. */
    if (!stmt->skipData)
        processed = refresh_matview_datafill(dest, dataQuery, queryString);

    /* Make the matview match the newly generated data. */
    if (concurrent)
    {
        int         old_depth = matview_maintenance_depth;

        PG_TRY();
        {
            refresh_by_match_merge(matviewOid, OIDNewHeap, relowner,
                                   save_sec_context);
        }
        PG_CATCH();
        {
            matview_maintenance_depth = old_depth;
            PG_RE_THROW();
        }
        PG_END_TRY();
        Assert(matview_maintenance_depth == old_depth);
    }
    else
    {
        refresh_by_heap_swap(matviewOid, OIDNewHeap, relpersistence);

        /*
         * Inform stats collector about our activity: basically, we truncated
         * the matview and inserted some new data.  (The concurrent code path
         * above doesn't need to worry about this because the inserts and
         * deletes it issues get counted by lower-level code.)
         */
        pgstat_count_truncate(matviewRel);
        if (!stmt->skipData)
            pgstat_count_heap_insert(matviewRel, processed);
    }

    table_close(matviewRel, NoLock);

    /* Roll back any GUC changes */
    AtEOXact_GUC(false, save_nestlevel);

    /* Restore userid and security context */
    SetUserIdAndSecContext(save_userid, save_sec_context);

    ObjectAddressSet(address, RelationRelationId, matviewOid);

    /*
     * Save the rowcount so that pg_stat_statements can track the total number
     * of rows processed by REFRESH MATERIALIZED VIEW command. Note that we
     * still don't display the rowcount in the command completion tag output,
     * i.e., the display_rowcount flag of CMDTAG_REFRESH_MATERIALIZED_VIEW
     * command tag is left false in cmdtaglist.h. Otherwise, the change of
     * completion tag output might break applications using it.
     */
    if (qc)
        SetQueryCompletion(qc, CMDTAG_REFRESH_MATERIALIZED_VIEW, processed);

    return address;
}

/*
 * refresh_matview_datafill
 *
 * Execute the given query, sending result rows to "dest" (which will
 * insert them into the target matview).
 *
 * Returns number of rows inserted.
 */
static uint64
refresh_matview_datafill(DestReceiver *dest, Query *query,
                         const char *queryString)
{
    List       *rewritten;
    PlannedStmt *plan;
    QueryDesc  *queryDesc;
    Query      *copied_query;
    uint64      processed;

    /* Lock and rewrite, using a copy to preserve the original query. */
    copied_query = copyObject(query);
    AcquireRewriteLocks(copied_query, true, false);
    rewritten = QueryRewrite(copied_query);

    /* SELECT should never rewrite to more or less than one SELECT query */
    if (list_length(rewritten) != 1)
        elog(ERROR, "unexpected rewrite result for REFRESH MATERIALIZED VIEW");
    query = (Query *) linitial(rewritten);

    /* Check for user-requested abort. */
    CHECK_FOR_INTERRUPTS();

    /* Plan the query which will generate data for the refresh. */
    plan = pg_plan_query(query, queryString, CURSOR_OPT_PARALLEL_OK, NULL);

    /*
     * Use a snapshot with an updated command ID to ensure this query sees
     * results of any previously executed queries.  (This could only matter if
     * the planner executed an allegedly-stable function that changed the
     * database contents, but let's do it anyway to be safe.)
     */
    PushCopiedSnapshot(GetActiveSnapshot());
    UpdateActiveSnapshotCommandId();

    /* Create a QueryDesc, redirecting output to our tuple receiver */
    queryDesc = CreateQueryDesc(plan, queryString,
                                GetActiveSnapshot(), InvalidSnapshot,
                                dest, NULL, NULL, 0);

    /* call ExecutorStart to prepare the plan for execution */
    ExecutorStart(queryDesc, 0);

    /* run the plan */
    ExecutorRun(queryDesc, ForwardScanDirection, 0L, true);

    processed = queryDesc->estate->es_processed;

    /* and clean up */
    ExecutorFinish(queryDesc);
    ExecutorEnd(queryDesc);

    FreeQueryDesc(queryDesc);

    PopActiveSnapshot();

    return processed;
}

DestReceiver *
CreateTransientRelDestReceiver(Oid transientoid)
{
    DR_transientrel *self = (DR_transientrel *) palloc0(sizeof(DR_transientrel));

    self->pub.receiveSlot = transientrel_receive;
    self->pub.rStartup = transientrel_startup;
    self->pub.rShutdown = transientrel_shutdown;
    self->pub.rDestroy = transientrel_destroy;
    self->pub.mydest = DestTransientRel;
    self->transientoid = transientoid;

    return (DestReceiver *) self;
}

/*
 * transientrel_startup --- executor startup
 */
static void
transientrel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
{
    DR_transientrel *myState = (DR_transientrel *) self;
    Relation    transientrel;

    transientrel = table_open(myState->transientoid, NoLock);

    /*
     * Fill private fields of myState for use by later routines
     */
    myState->transientrel = transientrel;
    myState->output_cid = GetCurrentCommandId(true);
    myState->ti_options = TABLE_INSERT_SKIP_FSM | TABLE_INSERT_FROZEN;
    myState->bistate = GetBulkInsertState();

    /*
     * Valid smgr_targblock implies something already wrote to the relation.
     * This may be harmless, but this function hasn't planned for it.
     */
    Assert(RelationGetTargetBlock(transientrel) == InvalidBlockNumber);
}

/*
 * transientrel_receive --- receive one tuple
 */
static bool
transientrel_receive(TupleTableSlot *slot, DestReceiver *self)
{
    DR_transientrel *myState = (DR_transientrel *) self;

    /*
     * Note that the input slot might not be of the type of the target
     * relation. That's supported by table_tuple_insert(), but slightly less
     * efficient than inserting with the right slot - but the alternative
     * would be to copy into a slot of the right type, which would not be
     * cheap either. This also doesn't allow accessing per-AM data (say a
     * tuple's xmin), but since we don't do that here...
     */

    table_tuple_insert(myState->transientrel,
                       slot,
                       myState->output_cid,
                       myState->ti_options,
                       myState->bistate);

    /* We know this is a newly created relation, so there are no indexes */

    return true;
}

/*
 * transientrel_shutdown --- executor end
 */
static void
transientrel_shutdown(DestReceiver *self)
{
    DR_transientrel *myState = (DR_transientrel *) self;

    FreeBulkInsertState(myState->bistate);

    table_finish_bulk_insert(myState->transientrel, myState->ti_options);

    /* close transientrel, but keep lock until commit */
    table_close(myState->transientrel, NoLock);
    myState->transientrel = NULL;
}

/*
 * transientrel_destroy --- release DestReceiver object
 */
static void
transientrel_destroy(DestReceiver *self)
{
    pfree(self);
}


/*
 * Given a qualified temporary table name, append an underscore followed by
 * the given integer, to make a new table name based on the old one.
 *
 * This leaks memory through palloc(), which won't be cleaned up until the
 * current memory context is freed.
 */
static char *
make_temptable_name_n(char *tempname, int n)
{
    StringInfoData namebuf;

    initStringInfo(&namebuf);
    appendStringInfoString(&namebuf, tempname);
    appendStringInfo(&namebuf, "_%d", n);
    return namebuf.data;
}

/*
 * refresh_by_match_merge
 *
 * Refresh a materialized view with transactional semantics, while allowing
 * concurrent reads.
 *
 * This is called after a new version of the data has been created in a
 * temporary table.  It performs a full outer join against the old version of
 * the data, producing "diff" results.  This join cannot work if there are any
 * duplicated rows in either the old or new versions, in the sense that every
 * column would compare as equal between the two rows.  It does work correctly
 * in the face of rows which have at least one NULL value, with all non-NULL
 * columns equal.  The behavior of NULLs on equality tests and on UNIQUE
 * indexes turns out to be quite convenient here; the tests we need to make
 * are consistent with default behavior.  If there is at least one UNIQUE
 * index on the materialized view, we have exactly the guarantee we need.
 *
 * The temporary table used to hold the diff results contains just the TID of
 * the old record (if matched) and the ROW from the new table as a single
 * column of complex record type (if matched).
 *
 * Once we have the diff table, we perform set-based DELETE and INSERT
 * operations against the materialized view, and discard both temporary
 * tables.
 *
 * Everything from the generation of the new data to applying the differences
 * takes place under cover of an ExclusiveLock, since it seems as though we
 * would want to prohibit not only concurrent REFRESH operations, but also
 * incremental maintenance.  It also doesn't seem reasonable or safe to allow
 * SELECT FOR UPDATE or SELECT FOR SHARE on rows being updated or deleted by
 * this command.
 */
static void
refresh_by_match_merge(Oid matviewOid, Oid tempOid, Oid relowner,
                       int save_sec_context)
{
    StringInfoData querybuf;
    Relation    matviewRel;
    Relation    tempRel;
    char       *matviewname;
    char       *tempname;
    char       *diffname;
    TupleDesc   tupdesc;
    bool        foundUniqueIndex;
    List       *indexoidlist;
    ListCell   *indexoidscan;
    int16       relnatts;
    Oid        *opUsedForQual;

    initStringInfo(&querybuf);
    matviewRel = table_open(matviewOid, NoLock);
    matviewname = quote_qualified_identifier(get_namespace_name(RelationGetNamespace(matviewRel)),
                                             RelationGetRelationName(matviewRel));
    tempRel = table_open(tempOid, NoLock);
    tempname = quote_qualified_identifier(get_namespace_name(RelationGetNamespace(tempRel)),
                                          RelationGetRelationName(tempRel));
    diffname = make_temptable_name_n(tempname, 2);

    relnatts = RelationGetNumberOfAttributes(matviewRel);

    /* Open SPI context. */
    if (SPI_connect() != SPI_OK_CONNECT)
        elog(ERROR, "SPI_connect failed");

    /* Analyze the temp table with the new contents. */
    appendStringInfo(&querybuf, "ANALYZE %s", tempname);
    if (SPI_exec(querybuf.data, 0) != SPI_OK_UTILITY)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    /*
     * We need to ensure that there are not duplicate rows without NULLs in
     * the new data set before we can count on the "diff" results.  Check for
     * that in a way that allows showing the first duplicated row found.  Even
     * after we pass this test, a unique index on the materialized view may
     * find a duplicate key problem.
     */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf,
                     "SELECT _$newdata FROM %s _$newdata "
                     "WHERE _$newdata IS NOT NULL AND EXISTS "
                     "(SELECT 1 FROM %s _$newdata2 WHERE _$newdata2 IS NOT NULL "
                     "AND _$newdata2 OPERATOR(pg_catalog.*=) _$newdata "
                     "AND _$newdata2.ctid OPERATOR(pg_catalog.<>) "
                     "_$newdata.ctid)",
                     tempname, tempname);
    if (SPI_execute(querybuf.data, false, 1) != SPI_OK_SELECT)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);
    if (SPI_processed > 0)
    {
        /*
         * Note that this ereport() is returning data to the user.  Generally,
         * we would want to make sure that the user has been granted access to
         * this data.  However, REFRESH MAT VIEW is only able to be run by the
         * owner of the mat view (or a superuser) and therefore there is no
         * need to check for access to data in the mat view.
         */
        ereport(ERROR,
                (errcode(ERRCODE_CARDINALITY_VIOLATION),
                 errmsg("new data for materialized view \"%s\" contains duplicate rows without any null columns",
                        RelationGetRelationName(matviewRel)),
                 errdetail("Row: %s",
                           SPI_getvalue(SPI_tuptable->vals[0], SPI_tuptable->tupdesc, 1))));
    }

    SetUserIdAndSecContext(relowner,
                           save_sec_context | SECURITY_LOCAL_USERID_CHANGE);

    /* Start building the query for creating the diff table. */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf,
                     "CREATE TEMP TABLE %s AS "
                     "SELECT _$mv.ctid AS tid, _$newdata "
                     "FROM %s _$mv FULL JOIN %s _$newdata ON (",
                     diffname, matviewname, tempname);

    /*
     * Get the list of index OIDs for the table from the relcache, and look up
     * each one in the pg_index syscache.  We will test for equality on all
     * columns present in all unique indexes which only reference columns and
     * include all rows.
     */
    tupdesc = matviewRel->rd_att;
    opUsedForQual = (Oid *) palloc0(sizeof(Oid) * relnatts);
    foundUniqueIndex = false;

    indexoidlist = RelationGetIndexList(matviewRel);

    foreach(indexoidscan, indexoidlist)
    {
        Oid         indexoid = lfirst_oid(indexoidscan);
        Relation    indexRel;

        indexRel = index_open(indexoid, RowExclusiveLock);
        if (is_usable_unique_index(indexRel))
        {
            Form_pg_index indexStruct = indexRel->rd_index;
            int         indnkeyatts = indexStruct->indnkeyatts;
            oidvector  *indclass;
            Datum       indclassDatum;
            bool        isnull;
            int         i;

            /* Must get indclass the hard way. */
            indclassDatum = SysCacheGetAttr(INDEXRELID,
                                            indexRel->rd_indextuple,
                                            Anum_pg_index_indclass,
                                            &isnull);
            Assert(!isnull);
            indclass = (oidvector *) DatumGetPointer(indclassDatum);

            /* Add quals for all columns from this index. */
            for (i = 0; i < indnkeyatts; i++)
            {
                int         attnum = indexStruct->indkey.values[i];
                Oid         opclass = indclass->values[i];
                Form_pg_attribute attr = TupleDescAttr(tupdesc, attnum - 1);
                Oid         attrtype = attr->atttypid;
                HeapTuple   cla_ht;
                Form_pg_opclass cla_tup;
                Oid         opfamily;
                Oid         opcintype;
                Oid         op;
                const char *leftop;
                const char *rightop;

                /*
                 * Identify the equality operator associated with this index
                 * column.  First we need to look up the column's opclass.
                 */
                cla_ht = SearchSysCache1(CLAOID, ObjectIdGetDatum(opclass));
                if (!HeapTupleIsValid(cla_ht))
                    elog(ERROR, "cache lookup failed for opclass %u", opclass);
                cla_tup = (Form_pg_opclass) GETSTRUCT(cla_ht);
                Assert(cla_tup->opcmethod == BTREE_AM_OID);
                opfamily = cla_tup->opcfamily;
                opcintype = cla_tup->opcintype;
                ReleaseSysCache(cla_ht);

                op = get_opfamily_member(opfamily, opcintype, opcintype,
                                         BTEqualStrategyNumber);
                if (!OidIsValid(op))
                    elog(ERROR, "missing operator %d(%u,%u) in opfamily %u",
                         BTEqualStrategyNumber, opcintype, opcintype, opfamily);

                /*
                 * If we find the same column with the same equality semantics
                 * in more than one index, we only need to emit the equality
                 * clause once.
                 *
                 * Since we only remember the last equality operator, this
                 * code could be fooled into emitting duplicate clauses given
                 * multiple indexes with several different opclasses ... but
                 * that's so unlikely it doesn't seem worth spending extra
                 * code to avoid.
                 */
                if (opUsedForQual[attnum - 1] == op)
                    continue;
                opUsedForQual[attnum - 1] = op;

                /*
                 * Actually add the qual, ANDed with any others.
                 */
                if (foundUniqueIndex)
                    appendStringInfoString(&querybuf, " AND ");

                leftop = quote_qualified_identifier("_$newdata",
                                                    NameStr(attr->attname));
                rightop = quote_qualified_identifier("_$mv",
                                                     NameStr(attr->attname));

                generate_operator_clause(&querybuf,
                                         leftop, attrtype,
                                         op,
                                         rightop, attrtype);

                foundUniqueIndex = true;
            }
        }

        /* Keep the locks, since we're about to run DML which needs them. */
        index_close(indexRel, NoLock);
    }

    list_free(indexoidlist);

    /*
     * There must be at least one usable unique index on the matview.
     *
     * ExecRefreshMatView() checks that after taking the exclusive lock on the
     * matview. So at least one unique index is guaranteed to exist here
     * because the lock is still being held; so an Assert seems sufficient.
     */
    Assert(foundUniqueIndex);

    appendStringInfoString(&querybuf,
                           " AND _$newdata OPERATOR(pg_catalog.*=) _$mv) "
                           "WHERE _$newdata IS NULL OR _$mv IS NULL "
                           "ORDER BY tid");

    /* Create the temporary "diff" table. */
    if (SPI_exec(querybuf.data, 0) != SPI_OK_UTILITY)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    SetUserIdAndSecContext(relowner,
                           save_sec_context | SECURITY_RESTRICTED_OPERATION);

    /*
     * We have no further use for data from the "full-data" temp table, but we
     * must keep it around because its type is referenced from the diff table.
     */

    /* Analyze the diff table. */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf, "ANALYZE %s", diffname);
    if (SPI_exec(querybuf.data, 0) != SPI_OK_UTILITY)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    OpenMatViewIncrementalMaintenance();

    /* Deletes must come before inserts; do them first. */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf,
                     "DELETE FROM %s _$mv WHERE ctid OPERATOR(pg_catalog.=) ANY "
                     "(SELECT _$diff.tid FROM %s _$diff "
                     "WHERE _$diff.tid IS NOT NULL "
                     "AND _$diff._$newdata IS NULL)",
                     matviewname, diffname);
    if (SPI_exec(querybuf.data, 0) != SPI_OK_DELETE)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    /* Inserts go last. */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf,
                     "INSERT INTO %s SELECT (_$diff._$newdata).* "
                     "FROM %s _$diff WHERE tid IS NULL",
                     matviewname, diffname);
    if (SPI_exec(querybuf.data, 0) != SPI_OK_INSERT)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    /* We're done maintaining the materialized view. */
    CloseMatViewIncrementalMaintenance();
    table_close(tempRel, NoLock);
    table_close(matviewRel, NoLock);

    /* Clean up temp tables. */
    resetStringInfo(&querybuf);
    appendStringInfo(&querybuf, "DROP TABLE %s, %s", diffname, tempname);
    if (SPI_exec(querybuf.data, 0) != SPI_OK_UTILITY)
        elog(ERROR, "SPI_exec failed: %s", querybuf.data);

    /* Close SPI context. */
    if (SPI_finish() != SPI_OK_FINISH)
        elog(ERROR, "SPI_finish failed");
}

/*
 * Swap the physical files of the target and transient tables, then rebuild
 * the target's indexes and throw away the transient table.  Security context
 * swapping is handled by the called function, so it is not needed here.
 */
static void
refresh_by_heap_swap(Oid matviewOid, Oid OIDNewHeap, char relpersistence)
{
    finish_heap_swap(matviewOid, OIDNewHeap, false, false, true, true,
                     RecentXmin, ReadNextMultiXactId(), relpersistence);
}

/*
 * Check whether specified index is usable for match merge.
 */
static bool
is_usable_unique_index(Relation indexRel)
{
    Form_pg_index indexStruct = indexRel->rd_index;

    /*
     * Must be unique, valid, immediate, non-partial, and be defined over
     * plain user columns (not expressions).  We also require it to be a
     * btree.  Even if we had any other unique index kinds, we'd not know how
     * to identify the corresponding equality operator, nor could we be sure
     * that the planner could implement the required FULL JOIN with non-btree
     * operators.
     */
    if (indexStruct->indisunique &&
        indexStruct->indimmediate &&
        indexRel->rd_rel->relam == BTREE_AM_OID &&
        indexStruct->indisvalid &&
        RelationGetIndexPredicate(indexRel) == NIL &&
        indexStruct->indnatts > 0)
    {
        /*
         * The point of groveling through the index columns individually is to
         * reject both index expressions and system columns.  Currently,
         * matviews couldn't have OID columns so there's no way to create an
         * index on a system column; but maybe someday that wouldn't be true,
         * so let's be safe.
         */
        int         numatts = indexStruct->indnatts;
        int         i;

        for (i = 0; i < numatts; i++)
        {
            int         attnum = indexStruct->indkey.values[i];

            if (attnum <= 0)
                return false;
        }
        return true;
    }
    return false;
}


/*
 * This should be used to test whether the backend is in a context where it is
 * OK to allow DML statements to modify materialized views.  We only want to
 * allow that for internal code driven by the materialized view definition,
 * not for arbitrary user-supplied code.
 *
 * While the function names reflect the fact that their main intended use is
 * incremental maintenance of materialized views (in response to changes to
 * the data in referenced relations), they are initially used to allow REFRESH
 * without blocking concurrent reads.
 */
bool
MatViewIncrementalMaintenanceIsEnabled(void)
{
    return matview_maintenance_depth > 0;
}

static void
OpenMatViewIncrementalMaintenance(void)
{
    matview_maintenance_depth++;
}

static void
CloseMatViewIncrementalMaintenance(void)
{
    matview_maintenance_depth--;
    Assert(matview_maintenance_depth >= 0);
}