postgresql generate sequence with no gap

Question

I must / have to create unique ID for invoices. I have a table id and another column for this unique number. I use serialization isolation level. Using

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Answer 1

You either lock the table to inserts, and/or need to have retry code. There's no other option available. If you stop to think about what can happen with:

1. parallel processes rolling back
2. locks timing out

you'll see why.