Running concurrent txns and measuring the timings in Postgres

Hi,

Is there a standard procedure to execute two or more concurrent txns at the same time? I understand that if we want to run concurrent txns, we need to execute them from different psql sessions. But how do we make sure that they begin execution almost at the same time.

Also, I'm interested in measuring the time taken across all executing txns, i.e. the time from the start of the earliest txns till the end of the last txn.

Best,

-SB