It could refer to either a PL/pgSQL variable or a table column

Question

I have a function in plpgsql:

CREATE OR REPLACE FUNCTION core.date_bs_from_ad(date_in_ad date)
RETURNS character varying AS $$
BEGIN
  RETURN (SELECT date_in_bs 
          FROM core.date_conversion
          WHERE date_in_ad = $1);
END
$$ LANGUAGE plpgsql;

It is created with no errors, but when I use this function it through following error:

ERROR:  column reference "date_in_ad" is ambiguous
LINE 3:   WHERE date_in_ad = $1
                ^
DETAIL:  It could refer to either a PL/pgSQL variable or a table column.
QUERY:  SELECT (
        SELECT MAX(date_in_bs) FROM core.date_conversion
        WHERE date_in_ad = $1
    )
CONTEXT:  PL/pgSQL function core.date_bs_from_ad(date) line 3 at RETURN
********** Error **********

ERROR: column reference "date_in_ad" is ambiguous
SQL state: 42702
Detail: It could refer to either a PL/pgSQL variable or a table column.
Context: PL/pgSQL function core.date_bs_from_ad(date) line 3 at RETURN

Answer 1

In cases like these, where the code is simple straightforward enough, sometimes it is useful to rely on one of these special plpgsql commands at the start of the function text:

#variable_conflict error
#variable_conflict use_variable
#variable_conflict use_column

In this case, it would be used as follows:

CREATE OR REPLACE FUNCTION core.date_bs_from_ad(date_in_ad date)
  RETURNS character varying AS
$$
#variable_conflict use_column
BEGIN
    RETURN(
        SELECT date_in_bs FROM core.date_conversion
        WHERE date_in_ad = $1
    );
END
$$

This is especially useful for cases when the clash is not with the parameters, but rather with the output column names, such as this:

CREATE OR REPLACE FUNCTION core.date_bs_from_ad(p_date_in_ad date)
  RETURNS TABLE (date_in_bs character varying) AS
$$
BEGIN
    RETURN QUERY
        SELECT date_in_bs FROM core.date_conversion
        WHERE date_in_ad = p_date_in_ad;
END;
$$

The function above will fail because it the compiler cannot decide if date_in_bs is the output variable name or one of core.date_conversion's columns. For problems like these, the command #variable_conflict use_column can really help.