USN-6656-2: PostgreSQL vulnerability

USN-6656-1 fixed several vulnerabilities in PostgreSQL. This update provides
the corresponding updates for Ubuntu 16.04 LTS

Original advisory details:

It was discovered that PostgreSQL incorrectly handled dropping privileges
when handling REFRESH MATERIALIZED VIEW CONCURRENTLY commands. If a user or
automatic system were tricked into running a specially crafted command, a
remote attacker could possibly use this issue to execute arbitrary SQL
functions.

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