Improving App Speed with a Second Serial column in PostgreSQL - sql

Question

Edit: (Explanation added)
I have a table that stores some data, it has the structure indicated below.
id tparti_id orde desc
1 1 10 One thing
2 1 20 Another thing
3 1 30 Last task of the month
4 2 10 First of second month
5 2 20 Second and last of second month
6 3 10 First of third month
The orde field it's the the sequence of rows with the same tparti_id, this value it's used on our app to sort the contents given tparti_id, the user can reorder it changing the values in order.
The values stored came from a text file and are parsed by a CakePHP app.
When a new row it's inserted the next value in the sequence of orde is calculated by searching the current value in orde and adding 10 given an id, if no orde is found returns 10
public function nextOrden($tpid){
$sql = "select orde from tdpies where tparti_id =".$tpid." order by orde desc limit 1;";
$r = $this->query($sql,$cachequeries = false);
if ($r){
$res = $r[0][0]['orde'] + 10;
} else {
$res = 10;
}
return $res;
}
It's working ok when inserting few records, but when inserting thousands of records it's time consuming.
How can improve performance:
Using a trigger when a new record it's created?
Using a new sequence with some trickery inside?
Thanks

Answer 1

You seem to have an auto-incremented id so you could calculated orde when you query table rather tha when you create it:
select p.*, 10 * (row_number() over (partition by tparti_id order by id)) as orde
from tdpies p;
If you want to handle this on insert, then add an index on tdpies(tparti_id, orde). Inside the trigger, you'll have something like:
new.orde := (select coalesce(max(orde), 0) + 10
from tdpies p
where p.tparti_id = new.tparti_id
);

Answer 2

It can (and should) be done in SQL in instead of in PHP
$sql = "
select coalesce(max(orde), 0) + 10 as new_id
from tdpies
where tparti_id = $tpid
"
If max(orde) returns null when there is no matching tparti_id the coalesce function returns 0.
Notice that it is not necessary to concatenate $tpid as it will be evaluated inside a double quoted string.
It is better to do the insert in the same query
insert into tdpies (
id, tparti_id, orde, description
)
select
1, $tpid,
coalesce(max(orde), 0) + 10,
'something about months'
from tdpies
where tparti_id = $tpid
You need an unique constraint (tparti_id, orde) in order to keep the table integrity.