Prevent Select sql from select the same data from DB PHP

Question

I Have some question about the Seq in PHP. I create 1 tabel to maintain the sequence table, ex. table is sequence and the field is seq(start from 1) The sequence should be unique value.

The problem is. there is 2 process which is run parallel that will use it. and I got the error message that say duplicate value.

The question. How to lock the table from another select query?

this is my code

//select seq
$sqlSeq = "select seq from sequence for update";
$resultSeq = pg_query($sqlSeq);
$rowSeq = pg_fetch_assoc($resultSeq);
$seqCif = $rowSeq['seq'];

//INSERT
$sqlInsert1 ="insert into TEST (customer_id) values( '".$seqCif."')";

//UPDATE 
 $sqlInsert1 .= "update sequence set seq=seq+1;" ; 

Can you help me for this case? Many Thanks befor..

Answer 1

One simple way to make the operation atomic and to guarantee an incremental id without gaps is to turn the statement to an insert... select... query:

insert into test(customer_id)
select coalesce(max(customer_id), 0) + 1 from test