Re: [PATCH] Add pg_get_policy_ddl() function to reconstruct CREATE POLICY statement

Hi Akshay,

I tested v15 on current master (57f93af36f): builds, make check passes, and
basic reconstruction, pretty mode, NULL handling, and default-clause omission
all look good. Two things.

1) Back in the v3/v4 discussion you decided the ON  name should always
be schema-qualified for safety (the pg_get_triggerdef_worker thread with
jian he), and that pretty here only controls formatting, not schema (Phil's
point). That reasoning didn't reach the USING / WITH CHECK expressions,
though: object references inside them are still qualified only by the caller's
search_path, so they can lose their schema and rebind to a different object
when the DDL is replayed elsewhere:

    CREATE FUNCTION s1.f(int) RETURNS bool LANGUAGE sql AS 'SELECT $1 > 0';
    CREATE FUNCTION s2.f(int) RETURNS bool LANGUAGE sql AS 'SELECT $1 < 0';
    CREATE POLICY pf ON t2 USING (s1.f(a));

    SET search_path = public, s1;
    SELECT ddl FROM pg_get_policy_ddl('t2', 'pf') AS ddl;
    --  CREATE POLICY pf ON public.t2 USING (f(a));      -- s1. dropped

    SET search_path = public, s2;
    CREATE POLICY pf ON t2 USING (f(a));                 -- now s2.f,
opposite meaning

So within one statement the ON clause is always qualified but the expression
body isn't -- and as Phil noted, the pretty flag can't fix this, since
pg_get_expr qualifies by search_path visibility regardless of pretty.

Worth settling the contract, given where this function sits. The old
pg_get_viewdef / ruledef / indexdef functions are search_path-aware and leave
qualification to the caller (pg_dump sets search_path empty around them so the
output is portable). The new pg_get_*_ddl functions committed so far (role,
database, tablespace) are on global, schemaless objects, so the question never
came up -- pg_get_policy_ddl is the first of the family whose output embeds
schema-qualifiable references. Your own pg_get_table_ddl patch already hit
this and deparses under a controlled search_path (narrowed to pg_catalog), so
the most consistent fix is to do the same here (NewGUCNestLevel + set_config);
an empty search_path already yields the fully-qualified s1.f(a). If instead
the intent is to follow the caller's search_path, that's fine too, but it
should be documented (and SET search_path = '' noted as the way to get
portable DDL).

2) The doc calls the second parameter policy_name, but the actual argument is
policyname, so the documented named-argument call fails:

    SELECT * FROM pg_get_policy_ddl("table" => 't'::regclass,
policy_name => 'p_all');
    ERROR:  function pg_get_policy_ddl(table => regclass, policy_name
=> unknown) does not exist

Regards,
Rui