rasmus@mindplay.dk writes:
> It looks like constraints are being checked row-by-row while the udpate is
> happening?
This is documented somewhere ... ah, here, in the COMPATIBILITY section of
the CREATE TABLE reference page:
Non-deferred Uniqueness Constraints
When a UNIQUE or PRIMARY KEY constraint is not deferrable, PostgreSQL
checks for uniqueness immediately whenever a row is inserted or
modified. The SQL standard says that uniqueness should be enforced only
at the end of the statement; this makes a difference when, for example,
a single command updates multiple key values. To obtain
standard-compliant behavior, declare the constraint as DEFERRABLE but
not deferred (i.e., INITIALLY IMMEDIATE). Be aware that this can be
significantly slower than immediate uniqueness checking.
regards, tom lane
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