Prepare results in ambiguous function call

[SantjagoCorkez]

• asyncpg version: 0.18.3
• PostgreSQL version: 11
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: local
• Python version: 3.7
• Platform: linux
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: 0.28.5
• Can the issue be reproduced under both asyncio and
  uvloop?: yes

There's a query SELECT generate_series FROM generate_series($1, $2, $3); There're multiple versions of the same function generate_series defined for different argument types. So when you emit PREPARE STATEMENT command you must emit types for which the function given should be selected by the query planner. The query then becomes PREPARE STATEMENT {stmt_name} (type1, type2, type3) AS SELECT generate_series FROM generate_series($1, $2, $3);

And there's no way to make asyncpg to emit such a type-hinted prepared statement. So this kind of queries always fail with

asyncpg.exceptions.AmbiguousFunctionError: function generate_series(unknown, unknown, unknown) is not unique
HINT:  Could not choose a best candidate function. You might need to add explicit type casts.

There's even no arg in Connection.prepare() that could help providing such a type hint for CoreConnection._prepare()

[1st1]

Try SELECT generate_series FROM generate_series($1::int4, $2::int4, $3::int4);