Fixes CDB_UserDataSize failing due ERROR: relation "*" does not exist.

Adds new _CDB_total_relation_size function that handles nonexistent
tables and does fallback to size=0.

That function could be used to cache total relation size or query another
table view with a cached total relation size.

Fixes #108

test/CDB_QuotaTest.sql

@@ -13,6 +13,11 @@ SELECT CDB_CartodbfyTable('big');
 INSERT INTO big SELECT generate_series(1,2048);
 INSERT INTO big SELECT generate_series(1,2048);
 INSERT INTO big SELECT generate_series(1,2048);
+-- Test for #108: https://github.com/CartoDB/cartodb-postgresql/issues/108
+SELECT CDB_UserDataSize();
+SELECT cartodb._CDB_total_relation_size('public', 'big');
+SELECT cartodb._CDB_total_relation_size('public', 'nonexistent_table_name');
+-- END Test for #108
 SELECT CDB_SetUserQuotaInBytes(2);
 INSERT INTO big VALUES (1);
 SELECT CDB_SetUserQuotaInBytes(0);

test/CDB_QuotaTest_expect

@@ -8,6 +8,9 @@ ERROR:  Quota exceeded by 3.9990234375KB
 INSERT 0 2048
 INSERT 0 2048
 INSERT 0 2048
+581632
+1163264
+0
 2
 ERROR:  Quota exceeded by 567.998046875KB
 0