Currently, the error reporting for duplicate key value violation for unique
indexes does not contain useful information for users when debugging. This
commit backports two commits from PostgreSQL (shown below) and updates our
multikey tuplesort to use the same. Now the error will display the first
instance of a duplicate key violation.

Authors: Jimmy Yih and Abhijit Subramanya

commit b680ae4b
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Sat Aug 1 19:59:41 2009 +0000

    Improve unique-constraint-violation error messages to include the exact
    values being complained of.

    In passing, also remove the arbitrary length limitation in the similar
    error detail message for foreign key violations.

    Itagaki Takahiro

 Conflicts:
	contrib/citext/expected/citext.out
	contrib/citext/expected/citext_1.out
	src/backend/access/common/indextuple.c
	src/backend/access/index/genam.c
	src/backend/access/nbtree/nbtinsert.c
	src/backend/utils/adt/ri_triggers.c
	src/backend/utils/adt/ruleutils.c
	src/include/access/genam.h
	src/include/access/itup.h
	src/include/utils/builtins.h
	src/test/regress/expected/alter_table.out
	src/test/regress/expected/arrays.out
	src/test/regress/expected/create_index.out
	src/test/regress/expected/plpgsql.out
	src/test/regress/expected/transactions.out
	src/test/regress/expected/uuid.out
	src/test/regress/output/constraints.source
	src/test/regress/output/tablespace.source

commit 527f0ae3
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Sat Aug 1 20:59:17 2009 +0000

    Department of second thoughts: let's show the exact key during unique index
    build failures, too.  Refactor a bit more since that error message isn't
    spelled the same.

 Conflicts:
	src/backend/access/nbtree/nbtinsert.c
	src/backend/utils/sort/tuplesort.c
	src/test/regress/expected/alter_table.out
	src/test/regress/expected/create_index.out

the cleanup of EState

Users should use error log functionality for error handling in external tables
and during copy since it is more robust and reliable than the old error table
functionality.

This test takes a very long time, so by splitting it and running other tests
in parallel, we can hopefully utilize multiple CPUs better during regression
tests, and finish the tests earlier.

There's no big principle on where exactly I split this, but I did take care
that the new 'partition1' test case doesn't need different expected output
files for the ORCA and non-ORCA case. That always makes maintenance easier.

These tests query pg_locks and fail if there are concurrent transactions
holding locks. Isolate them to separate file, so that we can safely run
other tests in parallel.

Try to put tests into larger groups, and try to run long-running tests
in parallel. This utilizes multiple CPUs better, making the overall suite
finish faster. On my laptop, this reduced the overall runtime of the
greenplum_schedule (i.e. excluding the Postgres tests in parallel_schedule)
from about 4 minutes to 3 minutes.

Exchanging a default partition is currently not allowed because there is
no validation on the data being exchanged. This commit add a GUC to
enable this if the user chooses to do so.