Converting H2/MySQL query to Postgres/cockroach - mysql

Question

I want to convert following —admittedly bad— query from H2/MySQL to Postgres/cockroach:
SET #UPDATE_TRANSFER=
(select count(*) from transfer where id=‘+transfer_id+' and consumed=false)>0;
update balance_address set balance =
case when #UPDATE_TRANSFER then balance +
(select value from transaction where transfer_id=‘+id+' and t_index=0)
else balance end where address =
(select address from transaction where transfer_id=‘+id+' and t_index=0)
There are three tables involved in this query: balance_address, bundle, and transaction. The goal of the query is to update the overall balance when a fund transfer happens.
A transfer can have many transaction bundled together. For instance, let’s assume Paul has $20 in his account and he wants to send $3 to Jane. This will result in 4 transactions:
One that adds $3 into Jane’s account
One transaction that removes the $20 from Paul account
One transactions that changes Paul account to 0
One transaction that puts to remainder of Paul funds in a new address; still belonging to him.
Each of these transaction in the whole transfer bundle has an index and a value. As you see above. So the goal of this update query is to update Jane’s account.
The challenge is that this transfer can be processed by many servers in parallel and there is no distributed lock. So, if we naively process in parallel, each server will increment Jane’s account, leading to erroneous results.
To prevent this, the balance_address table has a column called consumed. The first server that updates the balance, sets the transfer to consumed=true. Other servers or threads can only update if consumed is false.
So, my goal is to 1) improve this query and 2) rewrite it to work with posters. Right now, the variable construct is not accepted already.
PS. I cannot change the data model.

Answer 1

CockroachDB doesn't have variables, but the #UPDATE_TRANSFER variable is only used once, so you can just substitute the subquery inline:
update balance_address set balance =
case
when (select count(*) from transfer where id=$1 and consumed=false)>0
then balance + (select value from transaction where transfer_id=$1 and t_index=0)
else balance
end
where address =
(select address from transaction where transfer_id=$1 and t_index=0)
But this doesn't set the consumed flag. The simplest way to do this is to make this a multi step transaction in your client application:
num_rows = txn.execute("UPDATE transfer SET consumed=true
WHERE id=$1 AND consumed=false", transfer_id)
if num_rows == 0: return
value, address = txn.query("SELECT value, address FROM transaction
WHERE transfer_id=$1 and t_index=0", transfer_id)
txn.execute("UPDATE balance_address SET balance = balance+$1
WHERE address = $2", value, address)
In PostgreSQL, I think you could get this into one big statement using common table expressions. However, CockroachDB 2.0 only supports a subset of CTEs, and I don't think it's possible to do this with a CTE in cockroach yet.