Re: [HACKERS] Why type coercion is not performed for parameters?

If I evaluate expression typename('literal'), then type coercion is performed and the function is successfully resolved, i.e.

    SELECT regnamespace('"pg_catalog"');

But if I want to prepare this query, I get the error:

    postgres=#  prepare foo as SELECT regnamespace($1);
    ERROR:  function regnamespace(unknown) does not exist
    LINE 1: prepare foo as SELECT regnamespace($1);

Certainly, I can explicitly specify parameter type:

    prepare foo (text) as SELECT regnamespace($1);

and it will work. But it is not always possible.

Why do I need it? I want to implement autoprepare.
My original intention was to let parse_analyze_varparams to infer type of parameters from the context.
But it is not always possible  and sometime leads to different behavior of query.
For example if the query:

     select count(*) from test_range_gist where ir @> 10;

is replaced with

     select count(*) from test_range_gist where ir @> $1;

then type of parameter will be int4range rather then int, which corresponds to the different operator.