statscmds.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * statscmds.c
 *    Commands for creating and altering extended statistics objects
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/statscmds.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "access/relation.h"
#include "access/relscan.h"
#include "access/table.h"
#include "catalog/catalog.h"
#include "catalog/dependency.h"
#include "catalog/indexing.h"
#include "catalog/namespace.h"
#include "catalog/objectaccess.h"
#include "catalog/pg_namespace.h"
#include "catalog/pg_statistic_ext.h"
#include "catalog/pg_statistic_ext_data.h"
#include "commands/comment.h"
#include "commands/defrem.h"
#include "miscadmin.h"
#include "statistics/statistics.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/inval.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/syscache.h"
#include "utils/typcache.h"


static char *ChooseExtendedStatisticName(const char *name1, const char *name2,
                                         const char *label, Oid namespaceid);
static char *ChooseExtendedStatisticNameAddition(List *exprs);


/* qsort comparator for the attnums in CreateStatistics */
static int
compare_int16(const void *a, const void *b)
{
    int         av = *(const int16 *) a;
    int         bv = *(const int16 *) b;

    /* this can't overflow if int is wider than int16 */
    return (av - bv);
}

/*
 *      CREATE STATISTICS
 */
ObjectAddress
CreateStatistics(CreateStatsStmt *stmt)
{
    int16       attnums[STATS_MAX_DIMENSIONS];
    int         numcols = 0;
    char       *namestr;
    NameData    stxname;
    Oid         statoid;
    Oid         namespaceId;
    Oid         stxowner = GetUserId();
    HeapTuple   htup;
    Datum       values[Natts_pg_statistic_ext];
    bool        nulls[Natts_pg_statistic_ext];
    Datum       datavalues[Natts_pg_statistic_ext_data];
    bool        datanulls[Natts_pg_statistic_ext_data];
    int2vector *stxkeys;
    Relation    statrel;
    Relation    datarel;
    Relation    rel = NULL;
    Oid         relid;
    ObjectAddress parentobject,
                myself;
    Datum       types[3];       /* one for each possible type of statistic */
    int         ntypes;
    ArrayType  *stxkind;
    bool        build_ndistinct;
    bool        build_dependencies;
    bool        build_mcv;
    bool        requested_type = false;
    int         i;
    ListCell   *cell;

    Assert(IsA(stmt, CreateStatsStmt));

    /*
     * Examine the FROM clause.  Currently, we only allow it to be a single
     * simple table, but later we'll probably allow multiple tables and JOIN
     * syntax.  The grammar is already prepared for that, so we have to check
     * here that what we got is what we can support.
     */
    if (list_length(stmt->relations) != 1)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("only a single relation is allowed in CREATE STATISTICS")));

    foreach(cell, stmt->relations)
    {
        Node       *rln = (Node *) lfirst(cell);

        if (!IsA(rln, RangeVar))
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("only a single relation is allowed in CREATE STATISTICS")));

        /*
         * CREATE STATISTICS will influence future execution plans but does
         * not interfere with currently executing plans.  So it should be
         * enough to take only ShareUpdateExclusiveLock on relation,
         * conflicting with ANALYZE and other DDL that sets statistical
         * information, but not with normal queries.
         */
        rel = relation_openrv((RangeVar *) rln, ShareUpdateExclusiveLock);

        /* Restrict to allowed relation types */
        if (rel->rd_rel->relkind != RELKIND_RELATION &&
            rel->rd_rel->relkind != RELKIND_MATVIEW &&
            rel->rd_rel->relkind != RELKIND_FOREIGN_TABLE &&
            rel->rd_rel->relkind != RELKIND_PARTITIONED_TABLE)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("relation \"%s\" is not a table, foreign table, or materialized view",
                            RelationGetRelationName(rel))));

        /* You must own the relation to create stats on it */
        if (!pg_class_ownercheck(RelationGetRelid(rel), stxowner))
            aclcheck_error(ACLCHECK_NOT_OWNER, get_relkind_objtype(rel->rd_rel->relkind),
                           RelationGetRelationName(rel));
    }

    Assert(rel);
    relid = RelationGetRelid(rel);

    /*
     * If the node has a name, split it up and determine creation namespace.
     * If not (a possibility not considered by the grammar, but one which can
     * occur via the "CREATE TABLE ... (LIKE)" command), then we put the
     * object in the same namespace as the relation, and cons up a name for
     * it.
     */
    if (stmt->defnames)
        namespaceId = QualifiedNameGetCreationNamespace(stmt->defnames,
                                                        &namestr);
    else
    {
        namespaceId = RelationGetNamespace(rel);
        namestr = ChooseExtendedStatisticName(RelationGetRelationName(rel),
                                              ChooseExtendedStatisticNameAddition(stmt->exprs),
                                              "stat",
                                              namespaceId);
    }
    namestrcpy(&stxname, namestr);

    /*
     * Deal with the possibility that the statistics object already exists.
     */
    if (SearchSysCacheExists2(STATEXTNAMENSP,
                              CStringGetDatum(namestr),
                              ObjectIdGetDatum(namespaceId)))
    {
        if (stmt->if_not_exists)
        {
            ereport(NOTICE,
                    (errcode(ERRCODE_DUPLICATE_OBJECT),
                     errmsg("statistics object \"%s\" already exists, skipping",
                            namestr)));
            relation_close(rel, NoLock);
            return InvalidObjectAddress;
        }

        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_OBJECT),
                 errmsg("statistics object \"%s\" already exists", namestr)));
    }

    /*
     * Currently, we only allow simple column references in the expression
     * list.  That will change someday, and again the grammar already supports
     * it so we have to enforce restrictions here.  For now, we can convert
     * the expression list to a simple array of attnums.  While at it, enforce
     * some constraints.
     */
    foreach(cell, stmt->exprs)
    {
        Node       *expr = (Node *) lfirst(cell);
        ColumnRef  *cref;
        char       *attname;
        HeapTuple   atttuple;
        Form_pg_attribute attForm;
        TypeCacheEntry *type;

        if (!IsA(expr, ColumnRef))
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("only simple column references are allowed in CREATE STATISTICS")));
        cref = (ColumnRef *) expr;

        if (list_length(cref->fields) != 1)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("only simple column references are allowed in CREATE STATISTICS")));
        attname = strVal((Value *) linitial(cref->fields));

        atttuple = SearchSysCacheAttName(relid, attname);
        if (!HeapTupleIsValid(atttuple))
            ereport(ERROR,
                    (errcode(ERRCODE_UNDEFINED_COLUMN),
                     errmsg("column \"%s\" does not exist",
                            attname)));
        attForm = (Form_pg_attribute) GETSTRUCT(atttuple);

        /* Disallow use of system attributes in extended stats */
        if (attForm->attnum <= 0)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("statistics creation on system columns is not supported")));

        /* Disallow data types without a less-than operator */
        type = lookup_type_cache(attForm->atttypid, TYPECACHE_LT_OPR);
        if (type->lt_opr == InvalidOid)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("column \"%s\" cannot be used in statistics because its type %s has no default btree operator class",
                            attname, format_type_be(attForm->atttypid))));

        /* Make sure no more than STATS_MAX_DIMENSIONS columns are used */
        if (numcols >= STATS_MAX_DIMENSIONS)
            ereport(ERROR,
                    (errcode(ERRCODE_TOO_MANY_COLUMNS),
                     errmsg("cannot have more than %d columns in statistics",
                            STATS_MAX_DIMENSIONS)));

        attnums[numcols] = attForm->attnum;
        numcols++;
        ReleaseSysCache(atttuple);
    }

    /*
     * Check that at least two columns were specified in the statement. The
     * upper bound was already checked in the loop above.
     */
    if (numcols < 2)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
                 errmsg("extended statistics require at least 2 columns")));

    /*
     * Sort the attnums, which makes detecting duplicates somewhat easier, and
     * it does not hurt (it does not affect the efficiency, unlike for
     * indexes, for example).
     */
    qsort(attnums, numcols, sizeof(int16), compare_int16);

    /*
     * Check for duplicates in the list of columns. The attnums are sorted so
     * just check consecutive elements.
     */
    for (i = 1; i < numcols; i++)
    {
        if (attnums[i] == attnums[i - 1])
            ereport(ERROR,
                    (errcode(ERRCODE_DUPLICATE_COLUMN),
                     errmsg("duplicate column name in statistics definition")));
    }

    /* Form an int2vector representation of the sorted column list */
    stxkeys = buildint2vector(attnums, numcols);

    /*
     * Parse the statistics kinds.
     */
    build_ndistinct = false;
    build_dependencies = false;
    build_mcv = false;
    foreach(cell, stmt->stat_types)
    {
        char       *type = strVal((Value *) lfirst(cell));

        if (strcmp(type, "ndistinct") == 0)
        {
            build_ndistinct = true;
            requested_type = true;
        }
        else if (strcmp(type, "dependencies") == 0)
        {
            build_dependencies = true;
            requested_type = true;
        }
        else if (strcmp(type, "mcv") == 0)
        {
            build_mcv = true;
            requested_type = true;
        }
        else
            ereport(ERROR,
                    (errcode(ERRCODE_SYNTAX_ERROR),
                     errmsg("unrecognized statistics kind \"%s\"",
                            type)));
    }
    /* If no statistic type was specified, build them all. */
    if (!requested_type)
    {
        build_ndistinct = true;
        build_dependencies = true;
        build_mcv = true;
    }

    /* construct the char array of enabled statistic types */
    ntypes = 0;
    if (build_ndistinct)
        types[ntypes++] = CharGetDatum(STATS_EXT_NDISTINCT);
    if (build_dependencies)
        types[ntypes++] = CharGetDatum(STATS_EXT_DEPENDENCIES);
    if (build_mcv)
        types[ntypes++] = CharGetDatum(STATS_EXT_MCV);
    Assert(ntypes > 0 && ntypes <= lengthof(types));
    stxkind = construct_array(types, ntypes, CHAROID, 1, true, TYPALIGN_CHAR);

    statrel = table_open(StatisticExtRelationId, RowExclusiveLock);

    /*
     * Everything seems fine, so let's build the pg_statistic_ext tuple.
     */
    memset(values, 0, sizeof(values));
    memset(nulls, false, sizeof(nulls));

    statoid = GetNewOidWithIndex(statrel, StatisticExtOidIndexId,
                                 Anum_pg_statistic_ext_oid);
    values[Anum_pg_statistic_ext_oid - 1] = ObjectIdGetDatum(statoid);
    values[Anum_pg_statistic_ext_stxrelid - 1] = ObjectIdGetDatum(relid);
    values[Anum_pg_statistic_ext_stxname - 1] = NameGetDatum(&stxname);
    values[Anum_pg_statistic_ext_stxnamespace - 1] = ObjectIdGetDatum(namespaceId);
    values[Anum_pg_statistic_ext_stxstattarget - 1] = Int32GetDatum(-1);
    values[Anum_pg_statistic_ext_stxowner - 1] = ObjectIdGetDatum(stxowner);
    values[Anum_pg_statistic_ext_stxkeys - 1] = PointerGetDatum(stxkeys);
    values[Anum_pg_statistic_ext_stxkind - 1] = PointerGetDatum(stxkind);

    /* insert it into pg_statistic_ext */
    htup = heap_form_tuple(statrel->rd_att, values, nulls);
    CatalogTupleInsert(statrel, htup);
    heap_freetuple(htup);

    relation_close(statrel, RowExclusiveLock);

    /*
     * Also build the pg_statistic_ext_data tuple, to hold the actual
     * statistics data.
     */
    datarel = table_open(StatisticExtDataRelationId, RowExclusiveLock);

    memset(datavalues, 0, sizeof(datavalues));
    memset(datanulls, false, sizeof(datanulls));

    datavalues[Anum_pg_statistic_ext_data_stxoid - 1] = ObjectIdGetDatum(statoid);

    /* no statistics built yet */
    datanulls[Anum_pg_statistic_ext_data_stxdndistinct - 1] = true;
    datanulls[Anum_pg_statistic_ext_data_stxddependencies - 1] = true;
    datanulls[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;

    /* insert it into pg_statistic_ext_data */
    htup = heap_form_tuple(datarel->rd_att, datavalues, datanulls);
    CatalogTupleInsert(datarel, htup);
    heap_freetuple(htup);

    relation_close(datarel, RowExclusiveLock);

    InvokeObjectPostCreateHook(StatisticExtRelationId, statoid, 0);

    /*
     * Invalidate relcache so that others see the new statistics object.
     */
    CacheInvalidateRelcache(rel);

    relation_close(rel, NoLock);

    /*
     * Add an AUTO dependency on each column used in the stats, so that the
     * stats object goes away if any or all of them get dropped.
     */
    ObjectAddressSet(myself, StatisticExtRelationId, statoid);

    for (i = 0; i < numcols; i++)
    {
        ObjectAddressSubSet(parentobject, RelationRelationId, relid, attnums[i]);
        recordDependencyOn(&myself, &parentobject, DEPENDENCY_AUTO);
    }

    /*
     * Also add dependencies on namespace and owner.  These are required
     * because the stats object might have a different namespace and/or owner
     * than the underlying table(s).
     */
    ObjectAddressSet(parentobject, NamespaceRelationId, namespaceId);
    recordDependencyOn(&myself, &parentobject, DEPENDENCY_NORMAL);

    recordDependencyOnOwner(StatisticExtRelationId, statoid, stxowner);

    /*
     * XXX probably there should be a recordDependencyOnCurrentExtension call
     * here too, but we'd have to add support for ALTER EXTENSION ADD/DROP
     * STATISTICS, which is more work than it seems worth.
     */

    /* Add any requested comment */
    if (stmt->stxcomment != NULL)
        CreateComments(statoid, StatisticExtRelationId, 0,
                       stmt->stxcomment);

    /* Return stats object's address */
    return myself;
}

/*
 *      ALTER STATISTICS
 */
ObjectAddress
AlterStatistics(AlterStatsStmt *stmt)
{
    Relation    rel;
    Oid         stxoid;
    HeapTuple   oldtup;
    HeapTuple   newtup;
    Datum       repl_val[Natts_pg_statistic_ext];
    bool        repl_null[Natts_pg_statistic_ext];
    bool        repl_repl[Natts_pg_statistic_ext];
    ObjectAddress address;
    int         newtarget = stmt->stxstattarget;

    /* Limit statistics target to a sane range */
    if (newtarget < -1)
    {
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("statistics target %d is too low",
                        newtarget)));
    }
    else if (newtarget > 10000)
    {
        newtarget = 10000;
        ereport(WARNING,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("lowering statistics target to %d",
                        newtarget)));
    }

    /* lookup OID of the statistics object */
    stxoid = get_statistics_object_oid(stmt->defnames, stmt->missing_ok);

    /*
     * If we got here and the OID is not valid, it means the statistics does
     * not exist, but the command specified IF EXISTS. So report this as a
     * simple NOTICE and we're done.
     */
    if (!OidIsValid(stxoid))
    {
        char       *schemaname;
        char       *statname;

        Assert(stmt->missing_ok);

        DeconstructQualifiedName(stmt->defnames, &schemaname, &statname);

        if (schemaname)
            ereport(NOTICE,
                    (errmsg("statistics object \"%s.%s\" does not exist, skipping",
                            schemaname, statname)));
        else
            ereport(NOTICE,
                    (errmsg("statistics object \"%s\" does not exist, skipping",
                            statname)));

        return InvalidObjectAddress;
    }

    /* Search pg_statistic_ext */
    rel = table_open(StatisticExtRelationId, RowExclusiveLock);

    oldtup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(stxoid));

    /* Must be owner of the existing statistics object */
    if (!pg_statistics_object_ownercheck(stxoid, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_STATISTIC_EXT,
                       NameListToString(stmt->defnames));

    /* Build new tuple. */
    memset(repl_val, 0, sizeof(repl_val));
    memset(repl_null, false, sizeof(repl_null));
    memset(repl_repl, false, sizeof(repl_repl));

    /* replace the stxstattarget column */
    repl_repl[Anum_pg_statistic_ext_stxstattarget - 1] = true;
    repl_val[Anum_pg_statistic_ext_stxstattarget - 1] = Int32GetDatum(newtarget);

    newtup = heap_modify_tuple(oldtup, RelationGetDescr(rel),
                               repl_val, repl_null, repl_repl);

    /* Update system catalog. */
    CatalogTupleUpdate(rel, &newtup->t_self, newtup);

    InvokeObjectPostAlterHook(StatisticExtRelationId, stxoid, 0);

    ObjectAddressSet(address, StatisticExtRelationId, stxoid);

    /*
     * NOTE: because we only support altering the statistics target, not the
     * other fields, there is no need to update dependencies.
     */

    heap_freetuple(newtup);
    ReleaseSysCache(oldtup);

    table_close(rel, RowExclusiveLock);

    return address;
}

/*
 * Guts of statistics object deletion.
 */
void
RemoveStatisticsById(Oid statsOid)
{
    Relation    relation;
    HeapTuple   tup;
    Form_pg_statistic_ext statext;
    Oid         relid;

    /*
     * First delete the pg_statistic_ext_data tuple holding the actual
     * statistical data.
     */
    relation = table_open(StatisticExtDataRelationId, RowExclusiveLock);

    tup = SearchSysCache1(STATEXTDATASTXOID, ObjectIdGetDatum(statsOid));

    if (!HeapTupleIsValid(tup)) /* should not happen */
        elog(ERROR, "cache lookup failed for statistics data %u", statsOid);

    CatalogTupleDelete(relation, &tup->t_self);

    ReleaseSysCache(tup);

    table_close(relation, RowExclusiveLock);

    /*
     * Delete the pg_statistic_ext tuple.  Also send out a cache inval on the
     * associated table, so that dependent plans will be rebuilt.
     */
    relation = table_open(StatisticExtRelationId, RowExclusiveLock);

    tup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(statsOid));

    if (!HeapTupleIsValid(tup)) /* should not happen */
        elog(ERROR, "cache lookup failed for statistics object %u", statsOid);

    statext = (Form_pg_statistic_ext) GETSTRUCT(tup);
    relid = statext->stxrelid;

    CacheInvalidateRelcacheByRelid(relid);

    CatalogTupleDelete(relation, &tup->t_self);

    ReleaseSysCache(tup);

    table_close(relation, RowExclusiveLock);
}

/*
 * Update a statistics object for ALTER COLUMN TYPE on a source column.
 *
 * This could throw an error if the type change can't be supported.
 * If it can be supported, but the stats must be recomputed, a likely choice
 * would be to set the relevant column(s) of the pg_statistic_ext_data tuple
 * to null until the next ANALYZE.  (Note that the type change hasn't actually
 * happened yet, so one option that's *not* on the table is to recompute
 * immediately.)
 *
 * For both ndistinct and functional-dependencies stats, the on-disk
 * representation is independent of the source column data types, and it is
 * plausible to assume that the old statistic values will still be good for
 * the new column contents.  (Obviously, if the ALTER COLUMN TYPE has a USING
 * expression that substantially alters the semantic meaning of the column
 * values, this assumption could fail.  But that seems like a corner case
 * that doesn't justify zapping the stats in common cases.)
 *
 * For MCV lists that's not the case, as those statistics store the datums
 * internally. In this case we simply reset the statistics value to NULL.
 *
 * Note that "type change" includes collation change, which means we can rely
 * on the MCV list being consistent with the collation info in pg_attribute
 * during estimation.
 */
void
UpdateStatisticsForTypeChange(Oid statsOid, Oid relationOid, int attnum,
                              Oid oldColumnType, Oid newColumnType)
{
    HeapTuple   stup,
                oldtup;

    Relation    rel;

    Datum       values[Natts_pg_statistic_ext_data];
    bool        nulls[Natts_pg_statistic_ext_data];
    bool        replaces[Natts_pg_statistic_ext_data];

    oldtup = SearchSysCache1(STATEXTDATASTXOID, ObjectIdGetDatum(statsOid));
    if (!HeapTupleIsValid(oldtup))
        elog(ERROR, "cache lookup failed for statistics object %u", statsOid);

    /*
     * When none of the defined statistics types contain datum values from the
     * table's columns then there's no need to reset the stats. Functional
     * dependencies and ndistinct stats should still hold true.
     */
    if (!statext_is_kind_built(oldtup, STATS_EXT_MCV))
    {
        ReleaseSysCache(oldtup);
        return;
    }

    /*
     * OK, we need to reset some statistics. So let's build the new tuple,
     * replacing the affected statistics types with NULL.
     */
    memset(nulls, 0, Natts_pg_statistic_ext_data * sizeof(bool));
    memset(replaces, 0, Natts_pg_statistic_ext_data * sizeof(bool));
    memset(values, 0, Natts_pg_statistic_ext_data * sizeof(Datum));

    replaces[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;
    nulls[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;

    rel = table_open(StatisticExtDataRelationId, RowExclusiveLock);

    /* replace the old tuple */
    stup = heap_modify_tuple(oldtup,
                             RelationGetDescr(rel),
                             values,
                             nulls,
                             replaces);

    ReleaseSysCache(oldtup);
    CatalogTupleUpdate(rel, &stup->t_self, stup);

    heap_freetuple(stup);

    table_close(rel, RowExclusiveLock);
}

/*
 * Select a nonconflicting name for a new statistics.
 *
 * name1, name2, and label are used the same way as for makeObjectName(),
 * except that the label can't be NULL; digits will be appended to the label
 * if needed to create a name that is unique within the specified namespace.
 *
 * Returns a palloc'd string.
 *
 * Note: it is theoretically possible to get a collision anyway, if someone
 * else chooses the same name concurrently.  This is fairly unlikely to be
 * a problem in practice, especially if one is holding a share update
 * exclusive lock on the relation identified by name1.  However, if choosing
 * multiple names within a single command, you'd better create the new object
 * and do CommandCounterIncrement before choosing the next one!
 */
static char *
ChooseExtendedStatisticName(const char *name1, const char *name2,
                            const char *label, Oid namespaceid)
{
    int         pass = 0;
    char       *stxname = NULL;
    char        modlabel[NAMEDATALEN];

    /* try the unmodified label first */
    StrNCpy(modlabel, label, sizeof(modlabel));

    for (;;)
    {
        Oid         existingstats;

        stxname = makeObjectName(name1, name2, modlabel);

        existingstats = GetSysCacheOid2(STATEXTNAMENSP, Anum_pg_statistic_ext_oid,
                                        PointerGetDatum(stxname),
                                        ObjectIdGetDatum(namespaceid));
        if (!OidIsValid(existingstats))
            break;

        /* found a conflict, so try a new name component */
        pfree(stxname);
        snprintf(modlabel, sizeof(modlabel), "%s%d", label, ++pass);
    }

    return stxname;
}

/*
 * Generate "name2" for a new statistics given the list of column names for it
 * This will be passed to ChooseExtendedStatisticName along with the parent
 * table name and a suitable label.
 *
 * We know that less than NAMEDATALEN characters will actually be used,
 * so we can truncate the result once we've generated that many.
 *
 * XXX see also ChooseForeignKeyConstraintNameAddition and
 * ChooseIndexNameAddition.
 */
static char *
ChooseExtendedStatisticNameAddition(List *exprs)
{
    char        buf[NAMEDATALEN * 2];
    int         buflen = 0;
    ListCell   *lc;

    buf[0] = '\0';
    foreach(lc, exprs)
    {
        ColumnRef  *cref = (ColumnRef *) lfirst(lc);
        const char *name;

        /* It should be one of these, but just skip if it happens not to be */
        if (!IsA(cref, ColumnRef))
            continue;

        name = strVal((Value *) linitial(cref->fields));

        if (buflen > 0)
            buf[buflen++] = '_';    /* insert _ between names */

        /*
         * At this point we have buflen <= NAMEDATALEN.  name should be less
         * than NAMEDATALEN already, but use strlcpy for paranoia.
         */
        strlcpy(buf + buflen, name, NAMEDATALEN);
        buflen += strlen(buf + buflen);
        if (buflen >= NAMEDATALEN)
            break;
    }
    return pstrdup(buf);
}