/*-------------------------------------------------------------------------
 *
 * 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);

	/*
	 * 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);
}