How to insert before update (Custom audit) - postgresql

Question

I would like execute insert before I update some rows, insert would contain information about which table is updated, which column is updated, what query is executed etc..
And I wrote something like this:
DO $$
DECLARE _new_product_id bigint := 1;
_update_by_product_id bigint:= 2;
-- BEGIN TRANSACTION
BEGIN
DECLARE product_sql_query text := 'UPDATE products SET product_id =' + _new_product_id + 'WHERE product_id =' + _update_by_product_id + ';'
INSERT INTO products_audit (ordinal_number, table_name, column_name, action_applied, query_executed, value_before_update, value_after_update, created_date )
VALUES (1, 'products', 'product_id', 'UPDATE', users_query, 1, 1, NOW());
-- END TRANSACTION
COMMIT;
END;
$$
But I'm receiving an syntax error which says:
ERROR: syntax error at or near "INSERT" LINE 9: INSERT INTO
products_audit (ordinal_number, table...
What's wrong here, I guess product_sql_query value is not correct?

Answer 1

The DECLARE block needs to go before the first BEGIN of the PL/pgSQL - which is not the same as a BEGIN TRANSACTION. You can't control transactions in an anonymous PL/pgSQL block.
It's unclear to me what you intend with the product_sql_query variable, but removing all the syntax errors, the block should look like this:
DO $$
DECLARE
_new_product_id bigint := 1;
_update_by_product_id bigint:= 2;
product_sql_query text;
BEGIN
product_sql_query := 'UPDATE products SET product_id = ' || _new_product_id::text ||' WHERE product_id = ' || _update_by_product_id::text;
INSERT INTO products_audit
(ordinal_number, table_name, column_name, action_applied, query_executed, value_before_update, value_after_update, created_date)
VALUES
(1, 'products', 'product_id', 'UPDATE', users_query, 1, 1, NOW());
END;
$$