Michael Glaesemann <grzm@xxxxxxxxxxxxx> writes:
> I thought that the unique key would be checked at the end of the update
> statement:
No, it's never worked that way. Each row is checked as it is inserted.
With an example like this, you may or may not get a failure depending on
chance physical ordering of rows.
This is not per spec, and I believe we have a TODO to fix it, but a fix
is far from trivial.
regards, tom lane
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
