If multiple schema is used in a query then this information can be extracted by parsing the query.
But when the schema is being accessed by setting the search path then it becomes hard to find with which schema
query belongs to in pg_stat_statements.
Even if you were to store the search_path as a separate field, there is no promise that the items in it have not changed since the query was added to pg_stat_statements. Your best bet is to schema-qualify your relations when writing your queries. Then your pg_stat_statement output will contain the information you want.
Note that you can use the queryid to figure out (with a little work) which schemas were used for particular queries:
CREATE SCHEMA a; CREATE TABLE a.foo (id int);
CREATE SCHEMA b; CREATE TABLE b.foo (id int);
SET search_path = a; select * from foo;
SET search_path = b; select * from foo;
RESET search_path;
select query, queryid from pg_stat_statements where query ~ 'select \* from foo';
query | queryid
-------------------+----------------------
select * from foo | 255924940643424438
select * from foo | -7783557204835816030
(2 rows)
greg=# explain verbose select * from a.foo;
QUERY PLAN
---------------------------------------------------------
Seq Scan on a.foo (cost=0.00..35.50 rows=2550 width=4)
Output: id
Query Identifier: 255924940643424438
(3 rows)
greg=# explain verbose select * from b.foo;
QUERY PLAN
---------------------------------------------------------
Seq Scan on b.foo (cost=0.00..35.50 rows=2550 width=4)
Output: id
Query Identifier: -7783557204835816030
Cheers,
Greg
