How to implement dynamic sql function in postgresql? - postgresql

Question

I am using PostgreSQL 11 version.
I want to implement a function that takes the layer name (table), column name, and id as parameters.
create or replace function test(layer_name anyelement, field_name anyelement, object_id text)
returns setof anyelement
language plpgsql
as $function$
begin
return query execute format('
select
*
from
%s
where
%s = cast($1 as int4)'
, pg_typeof(layer_name), pg_typeof(field_name))
using object_id;
end;
$function$
;
This is the code I've implemented and when I call the function I get an error.
What am I doing wrong?
Error querying database. Cause: org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY
### The error occurred while setting parameters
### SQL: select * from test(?, ?, ?)
### Cause: org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY
; bad SQL grammar []; nested exception is org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY}
org.springframework.jdbc.BadSqlGrammarException:
### Error querying database. Cause: org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY
### The error occurred while setting parameters
### SQL: select * from test(?, ?, ?)
### Cause: org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY
; bad SQL grammar []; nested exception is org.postgresql.util.PSQLException: ERROR: syntax error at or near "="
Where: PL/pgSQL function test(anyelement,anyelement,text) line 3 at RETURN QUERY

Answer 1

You have to change your function slightly
Instead of field_name anyelement use field_name text in parameter.
and in place of pg_typeof(field_name) use only field_name:
So your function definition will be:
create or replace function test(layer_name anyelement, field_name text, object_id text)
returns setof anyelement
language plpgsql
as $function$
begin
return query execute format('
select
*
from
%s
where
%s = cast($1 as int4)'
, pg_typeof(layer_name), field_name)
using object_id;
end;
$function$
;
Most important part is calling of the function:
select * from test(null::table_name,'field_name','2');
Please note that your field_name always should be integer type and object_id should be number only because you are casting it to integer.
DEMO