What the best choice for speed, but for security? It very important to know because I do not know how large this online database will be, and can be very unplesent to discover after one week from going uploaded that the aplication has crash.

Normalization of your database in a well manner will sort the problem.

commented: best answer! +3

rajarajan07 suggestion on proper NORMALIZATION is the most important and crucial fact when designing databases.

Improper or even ignoring normalization always leads to inflexible "bigger" tables which usually suffer from redundancy, inserting, and updating anomalies. To work around such anomalies noticeably additional programming efforts on plain php, c, cpp, java level is necessary. On such poor designed database creating efficient sql queries/selects is hardly feasible.

-- tesu

(I am involved in very lage database system consisting of quite a few thousands tables. Detractors always saying if these sea of tables had only seen some proper normalization, company would have been able to cut down programmer staff to 50 percent.)