Oracle SQL - check sql data size

Run an explain plan on the query

EXPLAIN PLAN FOR SELECT a.col1, b.col2, c.col1 FROM a INNER JOIN b ONb.a_id=a.id LEFT JOIN c ON c.b_id=b.id WHERE a.somecol='data'; 

Display the plan table output

The explain plan should give you an estimate on the number of bytes accessed

An easier and more accurate way to be to enable AUTOTRACE option from SQL*Plus, that should give you a similar figure

SET AUTOTRACE TRACEONLYSELECT a.col1, b.col2, c.col1FROM aINNER JOIN b on b.a_id=a.idLEFT JOIN c on c.b_id=b.idWHERE a.somecol='data';

That should give some stats:

Statistics----------------------------------------------------------          0  recursive calls          0  db block gets         44  consistent gets          0  physical reads          0  redo size      24849  bytes sent via SQL*Net to client        408  bytes received via SQL*Net from client          6  SQL*Net roundtrips to/from client          0  sorts (memory)          0  sorts (disk)         66  rows processed

You can get the length of all the data being received, and that even after the executions of query(no pre-assumptions).

LEN NUMBER := 0;FOR C_DATA IN (SELECT a.col1, b.col2, c.col1                        FROM a                        INNER JOIN b on b.a_id=a.id                        LEFT JOIN c on c.b_id=b.id                        WHERE a.somecol='data')LOOP LEN = LEN + LENGTH(C_DATA.col1) + LENGTH(C_DATA.col2) + LENGTH(C_DATA.col3);END LOOP;

PS: You have used same column name for both table "a" and "c" i.e.col1, which may throw error while assigning values into the cursor. To resolve this, the cursore would be declared explicitly or REF cursor is to be used.