pg_temp support in migrations

[maxhawkins]

One of my migrations uses a pg_temp table to migrate data from one table to another:

CREATE TABLE pg_temp.migrate (val INT);
INSERT INTO pg_temp.migrate (val) SELECT val FROM old;
INSERT INTO new (val) SELECT val FROM pg_temp.migrate;

When I run sqlc it says

schema.sql:2055:1: schema "pg_temp" does not exist

The pg_temp schema should implicitly exist when loading the schema.

[maxhawkins]

This issue regressed in the experimental parser