Tags: shardman.

...rgeAppend, a SubqueryScan nodes.
Tags: shardman.

Tags: shardman.

Replace find_ok assert with elog panic message.

Improve the elog panic message introduced in previous commit.
Fix ForeignScan estimation logic.

Switch from AQO_EXPLAIN define to the two user GUCS: aqo.details
and aqo.show_hash.
New ExplainOneNode hook allows to add info into explain of each node.

AQO supports push-down of trivial JOIN and JOIN with mergejoinable clauses.

short-living planner memory context.
Some indentations fixed.

Big commit. To avoiding UPDATE/UPDATE/DELETE/INSERT conflicts we
use user locks on a pair (query_hash, fspace_hash) to guarantee that
only one backend can do INSERT/UPDATE/DELETE of a aqo table row.
Also we use dirty snapshot to see concurrently inserted/updated tuples.
Such conflict may cause deadlocks on end-of-transaction waiting.
Currently, if we detected such conflict, we refuse from we ML-base
changing.