Index not working as I expect

[Olek Marianski]

Hi,

my problem is with indexing table. Let's say I have a table:

id(int PK AI),c1(varchar 100),c2(varchar 100),c3(varchar 100),c4(date),c5(text)

I put an index on c1,c2,c3,c4.

I query database: SELECT * FROM table WHERE c1='c1' AND c2='c2' AND c3='c3' ORDER BY c4 DESC LIMIT 0,30;

I expect it to use my index and with over 600k records give me results in less than seconds. However it uses index, when i 'explain' i get key_len = 990 and rows = 30.000.

Why mysql hava to analize like 30k rows if I put the index and I expect to get and analyze only 30 rows? Where am I wrong? I would be grateful for any suggestions.

Config if needed: based on my.medium.cnf, run on 512MB RAM, 1 GHz VPS.