Query with substring

[Marco Bonfatti]

Hi!
I've migrated a db (and query) from a DB2 server to a MySql server.

I had a query:
"SELECT * FROM item, author WHERE item.i_a_id = author.a_id AND item.i_title LIKE 'my_search_string' ORDER BY item.i_title FETCH FIRST 50 ROWS ONLY"

A person suggested me to convert it with:
"SELECT * FROM ITEM, AUTHOR WHERE ITEM.I_A_ID = AUTHOR.A_ID AND substring(soundex(ITEM.I_TITLE), 0, 4)=substring(soundex('my_search_string'), 0, 4) ORDER BY ITEM.I_TITLE LIMIT 50"

I know that the function "substring" accepts the pos parameter > 0... but that's the problem!!!

If I put in the WHERE condition
"substring(soundex(ITEM.I_TITLE), 1, 4) = substring(soundex('my_search_string'), 1, 4)"
there's no results; instead, if i put
"substring(soundex(ITEM.I_TITLE), 0, 4) = substring(soundex('my_search_string'), 0, 4)"
the query spends a lot of time (about 5 seconds) but returns something. What's going on? I'm stuck.

Bye
Marco