Best guess you are running into what is described here:
https://www.postgresql.org/docs/9.5/static/explicit-locking.html#LOCKING-DEADLOCKS
Both transactions are holding locks on rows in T1 that the other wants
also.
I may be missing something, but I am not sure why it is necessary to run
both sessions concurrently? Could you not do session1 and once it
completes then session2?
Sessions are running concurrently because of flexibility - they are two
different scheduled jobs launching at different times and performing
different set of operations.
Of course I can play with scheduling timings and make them not intersect
with each other (which I've done already btw), but that's only a temp
solution.
So how in PostgreSQL-world 2 or more transactions can update the same
table without deadlocking? I can't believe it's not possible, there must
be some sort of synchronization primitive. Does it support a "named
mutex" concept from a system-programming world? I bet there is something
more suitable.
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