I currently use pgsql 7.2.4 (but the following has also been seen on pgsql 7.3.3) with a transaction level set to "read committed". It do a lot of little tests to understand how concurrency control works. Let see this scenario:
We have a table named "test_count" and a field named "count" The table contains 1 entry with count=1
Client 1: BEGIN; SELECT count FROM test_count FOR UPDATE; --> returns the only entry "1" ...
Client 2 : BEGIN; SELECT count FROM test_count FOR UPDATE; --> this query is blocked, ok ...
We continue :
Client 1: INSERT INTO test_count VALUES (2); COMMIT;
Client 2: (after commit of client 1) [The select that was blocked is now free. But the result is the first row containing "1". I'm surprised by this result] SELECT count FROM test_count; --> now returns the two rows, on containing "1", the other containing "2" COMMIT;
So my question is : why the SELECT...FOR UPDATE of client 2, when unblocked, returns only one row, and a following SELECT in the same transaction returns two rows ? Is there a mechanisme I don't understand ?
