Using prepared statement in stored function

Question

I have a table in the database:

create table store (
    ...
    n_status        integer not null,
    t_tag           varchar(4)
    t_name          varchar,
    t_description   varchar,
    dt_modified     timestamp not null,
    ...
);

In my stored function I need to execute the same select against this table multiple times:

select * from store
where n_place_id = [different values]
and t_tag is not null
and n_status > 0
and (t_name ~* t_search or t_description ~* t_search)
order by dt_modified desc
limit n_max;

Here, t_search and n_max are parameters into the stored function. I thought it would make sense to use a prepared statement for this, but I'm running into strange problems. Here's what I have:

create or replace function fn_get_data(t_search varchar, n_max integer)
  returns setof store as
$body$
declare
    resulter        store%rowtype;
    mid             integer;
begin
    prepare statement prep_stmt(integer) as
        select *
          from store
         where n_place_id = $1
           and (t_name ~* t_search or t_description ~* t_search)
      order by dt_modified
         limit n_max;

    for mid in
        (select n_place_id from ... where ...)
    loop
        for resulter in
            execute prep_stmt(mid)
        loop
            return next resulter;
        end loop;
    end loop;
end;$body$
  language 'plpgsql' volatile;

However when I actually run the function with

select * from fn_get_data('', 30)

I receive this error:

ERROR:  column "t_search" does not exist
LINE 3:   and (t_name ~* t_search or t_description ~* t_search)
                         ^
QUERY:  prepare prep_stmt(integer) as
        select * from store where n_status > 0 and t_tag is not null and n_museum = $1
        and (t_name ~* t_search or t_description ~* t_search)
        order by dt_modified desc limit maxres_free

Ok, maybe it doesn't like external variables in the prepared statement, so I changed this to be

prepare prep_stmt(integer, varchar, integer) as
select * from store where n_status > 0 and t_tag is not null and n_museum = $1
and (t_name ~* $2 or t_description ~* $2)
order by dt_modified desc limit $3

...

for resulter in
    execute prep_stmt(mid, t_search, n_max)

...

This time I get a different error:

ERROR:  function prep_stmt(integer, character varying, integer) does not exist
LINE 1: SELECT prep_stmt(mid, t_search, n_max)
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
QUERY:  SELECT prep_stmt(mid, t_search, n_max)

What am I missing here?

EDIT I added the relevant table structure at the top.

Answer 1

Looks to me like the PL/PgSQL EXECUTE for dynamic SQL trumps the regular SQL EXECUTE for prepared statements.

Code:

create or replace function prep_test() returns void as $$
begin
    PREPARE do_something AS SELECT 1;
    EXECUTE do_something;
end;
$$ LANGUAGE 'plpgsql';

Test:

regress=# select prep_test(1);
ERROR:  column "do_something" does not exist
LINE 1: SELECT do_something
               ^
QUERY:  SELECT do_something
CONTEXT:  PL/pgSQL function "prep_test" line 4 at EXECUTE statement

outside PL/PgSQL it works fine:

regress=# EXECUTE do_something;
?column?
----------
        1
(1 row)

I'm not sure how you'd execute a prepared statement within PL/PgSQL.

Out of interest, why are you trying to use prepared statements within PL/PgSQL? Plans are prepared and cached for PL/PgSQL anyway, it happens automatically.