Disallow aggregate functions in UPDATE commands (unless within a sub-SELECT).
This is disallowed by the SQL spec because it doesn't have any very sensible interpretation. Historically Postgres has allowed it but behaved strangely. As of PG 8.1 a server crash is possible if the MIN/MAX index optimization gets applied; rather than try to "fix" that, it seems best to just enforce the spec restriction. Per report from Josh Drake and Alvaro Herrera.
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