SQL unique : manual check vs catch exception -

SQL unique : manual check vs catch exception -

i'm working on big database , i'm seeking can speed database. question : when have unique index on somes fields, faster between create select request check if it's ok, or seek anyway , grab exception if entry exists ?

i made researchs nil conclusive. thanks.

a manual check won't think does. (see below.)

if check first, every insert requires 2 round-trips database. might require serializable transactions.

and have trap errors anyway. duplicate value one thing can go wrong on insert; there lot of other things can go wrong.

i insert, , trap errors.

the point of select before insert determine whether value exists in database. can't rely on work. here's why.

open 2 terminal sessions (for example), , connect both database. table exists. it's empty.

create table test ( test_id serial primary key, test_email varchar(15) not null unique ); a: begin transaction; a: select test_email test test_email = 'a@b.com'; (0 rows) b: begin transaction; a: insert test (test_email) values ('a@b.com'); insert 0 1 b: select test_email test test_email = 'a@b.com'; (0 rows) b: insert test (test_email) values ('a@b.com'); (waiting lock) a: commit; b: error: duplicate key value violates unique constraint...

sql unique