PostgreSQL: Create foreign table with string and variable - postgresql

Question

My problem: I need to define foreign table dynamically and set different where conditions every time. I am doing this in function, but I am getting error which doesn't make sense to me during creation of the foreign table(via oracle_fdw).
Creation of foreign table that works:
CREATE FOREIGN TABLE MYFOREIGNTABLE
(
column1 int,
column2 text
)
SERVER fwdb
OPTIONS (table $$(
select
column1,
column2
from
table1
where
column3 = 5
and column4 = 'a'
)$$);
Now if I try to split the string for putting there my variables (instead of variable I left there number so anybody can try it), it stop working and I am getting error
[Code: 0, SQL State: 42601] ERROR: syntax error at or near "||"
CREATE FOREIGN TABLE MYFOREIGNTABLE
(
column1 int,
column2 text
)
SERVER fwdb
OPTIONS (table $$(
select
column1,
column2
from
table1
where
column3 = $$ || 5 || $$
and column4 = 'a'
)$$);
Just for sure I tried my string in select to make sure I didn't do any syntax mistake and it works no problem
select $$(
select
column1,
column2
from
table1
where
column3 = $$ || 5 || $$
and column4 = 'a'
)$$
I tried few other things like using concat() or putting my whole string into variable OPTIONS (table myvariable); But neither worked. What is the correct syntax here?
PostgreSQL 11.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5
20150623 (Red Hat 4.8.5-39), 64-bit

Answer 1

You have to use a string literal as value for a FDW option, expressions like the string concatenation you are trying to use are not allowed.
You will have to construct the complete statement with dynamic SQL, for example
DO
$$DECLARE
var integer := 5;
BEGIN
EXECUTE
format(
E'CREATE FOREIGN TABLE MYFOREIGNTABLE (\n'
' column1 int,\n'
' column2 text\n'
') SERVER fwdb OPTIONS (\n'
' table ''(SELECT column1,\n'
' column2\n'
' FROM table1\n'
' WHERE column3 = %s\n'
' AND column4 = ''''a'''')'')',
var
);
END;$$;
For string variables you have to get the quoting right by using quote_literal(quote_literal(var)).