Use catalog query to discover tables to process in vacuumdb

vacuumdb would use a catalog query only when the command caller does not
define a list of tables.  Switching to a catalog table represents two
advantages:
- Relation existence check can happen before running any VACUUM or
ANALYZE query.  Before this change, if multiple relations are defined
using --table, the utility would fail only after processing the
firstly-defined ones, which may be a long some depending on the size of
the relation.  This adds checks for the relation names, and does
nothing, at least yet, for the attribute names.
- More filtering options can become available for the utility user.
These options, which may be introduced later on, are based on the
relation size or the relation age, and need to be made available even if
the user does not list any specific table with --table.

Author: Nathan Bossart
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/FFE5373C-E26A-495B-B5C8-911EC4A41C5E@amazon.com

src/bin/scripts/common.c

@@ -265,9 +265,9 @@ executeMaintenanceCommand(PGconn *conn, const char *query, bool echo)
  * finish using them, pg_free(*table).  *columns is a pointer into "spec",
  * possibly to its NUL terminator.
  */
-static void
-split_table_columns_spec(const char *spec, int encoding,
-						 char **table, const char **columns)
+void
+splitTableColumnsSpec(const char *spec, int encoding,
+					  char **table, const char **columns)
 {
 	bool		inquotes = false;
 	const char *cp = spec;
@@ -318,7 +318,7 @@ appendQualifiedRelation(PQExpBuffer buf, const char *spec,
 		return;
 	}
 
-	split_table_columns_spec(spec, PQclientEncoding(conn), &table, &columns);
+	splitTableColumnsSpec(spec, PQclientEncoding(conn), &table, &columns);
 
 	/*
 	 * Query must remain ABSOLUTELY devoid of unqualified names.  This would

src/bin/scripts/common.h

@@ -48,6 +48,9 @@ extern void executeCommand(PGconn *conn, const char *query,
 extern bool executeMaintenanceCommand(PGconn *conn, const char *query,
 						  bool echo);
 
+extern void splitTableColumnsSpec(const char *spec, int encoding,
+					  char **table, const char **columns);
+
 extern void appendQualifiedRelation(PQExpBuffer buf, const char *name,
 						PGconn *conn, const char *progname, bool echo);