If your purpose in doing this is just to generate unique keys, you should be using sequences instead.
I would need 150 separate sequences, because each idsection needs its own, independent sequence of idthread.
Note that you probably want to lock the table before doing this or two transactions running at the same time can generate the same value for idthread.
That's a surprise. I could have made two separate queries (a select and then insert) in my programming language (Python), but I wanted to make it in one query PRECISELY because I thought that would prevent the race condition that you describe. Are you quite sure?
For example: SELECT coallesce(max(idthread),0)+1 FROM table WHERE idsection = 'CZE';
Someone already sent me this by private email, and it works fine.
If there is a compound index on idthread and idsection, then you are probably better off using something like the following to take advantage of the index: coallesce((SELECT idthread FROM table WHERE idsection = 'CZE' ORDER BY idthread DESC, idsection DESC LIMT 1))+1
That's interesting and valuable, thank you very much.
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