Count rows affected by DELETE

问题:

I use this code to verify the DELETE sentence, but I am sure you know a better way:

CREATE OR REPLACE FUNCTION my_schema.sp_delete_row_table(table_name character varying
                                                       , id_column character varying
                                                       , id_value integer)
  RETURNS integer AS
$BODY$
  DECLARE
    BEFORE_ROWS integer;
    AFTER_ROWS integer;
  BEGIN
    EXECUTE 'SELECT count(*) FROM ' || TABLE_NAME INTO BEFORE_ROWS;
    EXECUTE 'DELETE FROM ' || TABLE_NAME || ' WHERE ' || ID_COLUMN || ' = ' || (ID_VALUE)::varchar;
    EXECUTE 'SELECT count(*) FROM ' || TABLE_NAME INTO AFTER_ROWS;

    IF BEFORE_ROWS - AFTER_ROWS = 1 THEN
      RETURN 1;
    ELSE
      RETURN 2;
    END IF;
  EXCEPTION WHEN OTHERS THEN
    RETURN 0;
  END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

How to improve this code? I need it to work in Postgres 8.4, 9.1 and 9.2.

回答1:

Look into the variables called found and row_count:

http://www.postgresql.org/docs/current/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-DIAGNOSTICS

found is true if any rows were affected. row_count gives you the number of affected rows.

IF FOUND THEN
  GET DIAGNOSTICS integer_var = ROW_COUNT;
END IF;

回答2:

Actually, you cannot use FOUND with EXECUTE. The manual:

Note in particular that EXECUTE changes the output of GET DIAGNOSTICS, but does not change FOUND.

There are a couple of other things that might be improved. First of all, your original is open to SQL injection. I suggest:

CREATE OR REPLACE FUNCTION my_schema.sp_delete_row_table(table_name regclass
                                                       , id_column  text
                                                       , id_value   int
                                                       , OUT del_ct int) AS
$func$
BEGIN
   EXECUTE format ('DELETE FROM %s WHERE %I = $1', table_name, id_column);
   USING id_value;                     -- assuming integer columns

   GET DIAGNOSTICS del_ct = ROW_COUNT; -- directly assign OUT parameter

EXCEPTION WHEN OTHERS THEN
   del_ct := 0;
END
$func$  LANGUAGE plpgsql;

format() requires Postgres 9.1 or later. You can replace it with string concatenation, but be sure to use escape the column name properly with quote_ident()!
The rest works for 8.4 as well.

Closely related answers:

• Dynamic SQL (EXECUTE) as condition for IF statement
• Table name as a PostgreSQL function parameter