Adding new Duckdb read functions

* Added `read_parquet(path TEXT[])` function that can be used to read
  multiple parquet files. Argument for this function should be provided
  in PostgreSQL syntax. To read multiple parquet files function should
  be called with argument `ARRAY['...','...']`.

* Added `read_csv(path TEXT)` and `read_parquet(path TEXT[])` reading
  single and multiple CSV files.

quack--0.0.1.sql

@@ -4,6 +4,30 @@ CREATE OR REPLACE FUNCTION read_parquet(path text)
 RETURNS SETOF record LANGUAGE 'plpgsql' AS
 $func$
 BEGIN
-  RETURN QUERY EXECUTE 'SELECT 1';
+   RAISE EXCEPTION 'Function `read_parquet(TEXT)` only works with Duckdb execution.';
+END;
+$func$;
+
+CREATE OR REPLACE FUNCTION read_parquet(path text[])
+RETURNS SETOF record LANGUAGE 'plpgsql' AS
+$func$
+BEGIN
+   RAISE EXCEPTION 'Function `read_parquet(TEXT[])` only works with Duckdb execution.';
+END;
+$func$;
+
+CREATE OR REPLACE FUNCTION read_csv(path text)
+RETURNS SETOF record LANGUAGE 'plpgsql' AS
+$func$
+BEGIN
+   RAISE EXCEPTION 'Function `read_csv(TEXT)` only works with Duckdb execution.';
+END;
+$func$;
+
+CREATE OR REPLACE FUNCTION read_csv(path text[])
+RETURNS SETOF record LANGUAGE 'plpgsql' AS
+$func$
+BEGIN
+   RAISE EXCEPTION 'Function `read_csv(TEXT[])` only works with Duckdb execution.';
 END;
 $func$;