Function postgresAcquireSampleRowsFunc

contrib/postgres_fdw/postgres_fdw.c:5010–5164

* Acquire a random sample of rows from foreign table managed by postgres_fdw. * * We fetch the whole table from the remote side and pick out some sample rows. * * Selected rows are returned in the caller-allocated array rows[], * which must have at least targrows entries. * The actual number of rows selected is returned as the function result. * We also count the total number of rows in the

 * currently (the planner only pays attention to correlation for indexscans).
 */
static int
postgresAcquireSampleRowsFunc(Relation relation, int elevel,
							  HeapTuple *rows, int targrows,
							  double *totalrows,
							  double *totaldeadrows)
{
	PgFdwAnalyzeState astate;
	ForeignTable *table;
	ForeignServer *server;
	UserMapping *user;
	PGconn	   *conn;
	unsigned int cursor_number;
	StringInfoData sql;
	PGresult   *volatile res = NULL;

	/* Initialize workspace state */
	astate.rel = relation;
	astate.attinmeta = TupleDescGetAttInMetadata(RelationGetDescr(relation));

	astate.rows = rows;
	astate.targrows = targrows;
	astate.numrows = 0;
	astate.samplerows = 0;
	astate.rowstoskip = -1;		/* -1 means not set yet */
	reservoir_init_selection_state(&astate.rstate, targrows);

	/* Remember ANALYZE context, and create a per-tuple temp context */
	astate.anl_cxt = CurrentMemoryContext;
	astate.temp_cxt = AllocSetContextCreate(CurrentMemoryContext,
											"postgres_fdw temporary data",
											ALLOCSET_SMALL_SIZES);

	/*
	 * Get the connection to use.  We do the remote access as the table's
	 * owner, even if the ANALYZE was started by some other user.
	 */
	table = GetForeignTable(RelationGetRelid(relation));
	server = GetForeignServer(table->serverid);
	user = GetUserMapping(relation->rd_rel->relowner, table->serverid);
	conn = GetConnection(user, false, NULL);

	/*
	 * Construct cursor that retrieves whole rows from remote.
	 */
	cursor_number = GetCursorNumber(conn);
	initStringInfo(&sql);
	appendStringInfo(&sql, "DECLARE c%u CURSOR FOR ", cursor_number);
	deparseAnalyzeSql(&sql, relation, &astate.retrieved_attrs);

	/* In what follows, do not risk leaking any PGresults. */
	PG_TRY();
	{
		char		fetch_sql[64];
		int			fetch_size;
		ListCell   *lc;

		res = pgfdw_exec_query(conn, sql.data, NULL);
		if (PQresultStatus(res) != PGRES_COMMAND_OK)