Function 2 – Computing the size of a table without locks

Here is a function that does what pg_relation_size does, more or less, without taking any locks. Because of this, it is always fast, but it may give an incorrect result if the table is being heavily altered at the same time:

CREATE OR REPLACE FUNCTION pg_relation_size_nolock(tablename regclass) 
RETURNS BIGINT 
LANGUAGE plpgsql 
AS $$ 
DECLARE 
    classoutput RECORD; 
    tsid        INTEGER; 
    rid         INTEGER; 
    dbid        INTEGER; 
    filepath    TEXT; 
    filename    TEXT; 
    datadir     TEXT; 
    i           INTEGER := 0; 
    tablesize   BIGINT; 
BEGIN 
    -- 
    -- Get data directory 
    -- 
    EXECUTE 'SHOW data_directory' INTO datadir; 
    -- 
    -- Get relfilenode and reltablespace 
    -- 
    SELECT 
     reltablespace as tsid 
    ,relfilenode as rid 
    INTO classoutput 
    FROM pg_class 
    WHERE oid = tablename 
    AND relkind = 'r'; 
    -- 
    -- Throw an error if we can't find the tablename specified 
    -- 
    IF NOT FOUND THEN 
      RAISE EXCEPTION 'tablename % not found', tablename; 
    END IF; 
    tsid := classoutput.tsid; 
    rid := classoutput.rid; 
    -- 
    -- Get the database object identifier (oid) 
    -- 
    SELECT oid INTO dbid 
    FROM pg_database 
    WHERE datname = current_database(); 
    -- 
    -- Use some internals knowledge to set the filepath 
    -- 
    IF tsid = 0 THEN 
  filepath := datadir || '/base/' || dbid || '/' || rid; 
    ELSE 
  filepath := datadir || '/pg_tblspc/' || tsid || '/' 
              || dbid || '/' || rid; 
    END IF; 
    -- 
    -- Look for the first file. Report if missing 
    -- 
    SELECT (pg_stat_file(filepath)).size 
    INTO tablesize; 
    -- 
    -- Sum the sizes of additional files, if any 
    -- 
    WHILE FOUND LOOP 
  i := i + 1; 
  filename := filepath || '.' || i; 
        -- 
        -- pg_stat_file returns ERROR if it cannot see file 
        -- so we must trap the error and exit loop 
        -- 
      BEGIN 
      SELECT tablesize + (pg_stat_file(filename)).size 
          INTO tablesize; 
      EXCEPTION 
           WHEN OTHERS THEN 
          EXIT; 
      END; 
    END LOOP; 
    RETURN tablesize; 
END; 
$$;

This function can also work on Windows with a few minor changes, which are left as an exercise for you.