Dynamic table name for INSERT INTO query - postgresql

Question

I am trying to figure out how to write an INSERT INTO query with table name and column name of the source as parameter.
For starters I was just trying to parametrize the source table name. I have written the following query. For now I am declaring and assigning the value of the variable tablename directly, but in actual example it would come from some other source/list. The target table has only one column.
CREATE OR REPLACE FUNCTION foo()
RETURNS void AS
$$
DECLARE
tablename text;
BEGIN
tablename := 'Table_1';
EXECUTE 'INSERT INTO "Schemaname"."targettable"
SELECT "Col_A"
FROM "schemaname".'
||quote_ident(tablename);
END
$$ LANGUAGE PLPGSQL;
Although the query runs without any error no changes are reflected at the target table. On running the query I get the following output.
Query OK, 0 rows affected (execution time: 296 ms; total time: 296 ms)
I want the changes to be reflected at the target table. I don't know how to resolve the problem.

Answer 1

Audited code
CREATE OR REPLACE FUNCTION foo()
RETURNS void AS
$func$
DECLARE
_tbl text := 'Table_1'; -- or 'table_1'?
BEGIN
EXECUTE 'INSERT INTO schemaname.targettable(column_name)
SELECT "Col_A"
FROM schemaname.' || quote_ident(_tbl); -- or "Schemaname"?
END
$func$ LANGUAGE plpgsql;
Always use an explicit target list for persisted INSERT statements.
You can assign variables at declare time.
It's a wide-spread folly to use double-quoted identifiers to preserve otherwise illegal spelling. You have to keep double-quoting the name for the rest of its existence. One or more of those errors seem to have crept into your code: "Schemaname" or "schemaname"? Table_1 or "Table_1"?
Are PostgreSQL column names case-sensitive?
When you provide an identifier like a table name as text parameter and escape it with quote_ident(), it is case sensitive!
Identifiers in SQL code are cast to lower case unless double-quoted. But quote-ident() (which you must use to defend against SQL injection) preserves the spelling you provide with double-quotes where necessary.
Function with parameter
CREATE OR REPLACE FUNCTION foo(_tbl text)
RETURNS void AS
$func$
BEGIN
EXECUTE 'INSERT INTO schemaname.targettable(column_name)
SELECT "Col_A"
FROM schemaname.' || quote_ident(_tbl);
END
$func$ LANGUAGE plpgsql;
Call:
SELECT foo('tablename'); -- tablename is case sensitive
There are other ways:
Table name as a PostgreSQL function parameter