[Starbish]

why many of the plugins call SQL_TConnect on every map change?

is it needed to get working well?

i think once is enough to get that function work..

[Bacardi]

In which plugins ?

Yes, you are right about connecting once to database, then make those necessary things you like.
If connect multiple times in database withouth closing previous ones, it will leave many "open" connections to mysql. (until unload that specific plugin)

Depends plugin activity, it's maybe better connect temporary to dataabse and close it:
connect to database -> use/query/update etc. etc. -> close database connection.
- This should make sure, you are or not getting connection to that database. Never know what block you out.

...than connect once on plugin load and keep it open that same connection all the time:
plugin load connect db -> use/query/update etc. etc. -> plugin unload close db
- Now if connection suddenly lost to database, your plugin start getting errors.