IMPALA-4340: explain how to install postgresql-9.5 or higher

The random query generator needs to compare against PostgresQL 9.5 or
higher to take advantage of some of the more recent features, especially
as it pertains to Impala/Kudu INSERT and UPSERT queries.  Developers
will need assistance setting up their development environments if they
need to use the random query generator.

This patch provides instructions on how to do so. We provide
instructions, not automation, since this will have side-effects on
developers' workstations: we can't presume to know how a developer might
want to install or configure postgres, and we haven't tested on anything
except our own development environment.

Change-Id: I1e3b510120451fcb5af97145fa47ccb4c53f00d9
Reviewed-on: http://gerrit.cloudera.org:8080/4846
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Tim Armstrong <tarmstrong@cloudera.com>

tests/comparison/POSTGRES.txt

@@ -0,0 +1,84 @@
+Motivation
+----------
+To test Impala/Kudu:
+  1. INSERT with ignorable primary key violations
+  2. "UPSERT"
+
+we need PostgresQL 9.5 or higher.
+
+https://www.postgresql.org/docs/9.5/static/sql-insert.html
+
+PostgresQL 9.5+ Installation
+----------------------------
+
+Common GNU/Linux distributions won't necessarily have PostgresQL as high
+as 9.5 available. Fortunately, PostgresQL understands this and provides
+instructions for adding sources for both Debian- and Redhat-based
+packages.
+
+https://wiki.postgresql.org/wiki/Apt
+https://yum.postgresql.org/repopackages.php
+
+What follows are instructions for getting this working on Ubuntu 14.04.
+
+Use the instructions at https://wiki.postgresql.org/wiki/Apt
+
+Note: apt pinning is possible, but if you already installed postgres
+from your standard GNU/Linux sources, you'll be forced to upgrade some
+packages like postgresql-common anyway. Resolving those problems is out
+of the scope of this document.
+
+Note: If you have postgresql-9.3 installed, your 9.3 installation will
+get updated to the latest point release version.
+
+If you then install a new PostgresQL version, you might be prompted what
+to do about /etc/postgresql-common/createcluster.conf . I elected to
+keep the local version installed.
+
+Before you start your new version of PostgresQL, you will probably want
+to make it easier to connect to the server:
+
+  $ sudo perl -pi -e 's/(?:peer|md5)/trust/g' /etc/postgresql/9.5/main/pg_hba.conf
+
+Note: By default, PostgresQL servers only bind to localhost, so the
+command above should only affect the localhost TCP server.
+
+Finally, you can 'sudo service postgresql start'. PostgresQL 9.3 is
+still listening on port 5432, but note 9.5 is also running, on 5433.
+
+PostgresQL 9.5+ Usage
+---------------------
+
+Databases aren't shared across versions. You'll need to load data, e.g.
+via:
+
+  $ tests/comparison/data_generator.py \
+      --use-postgresql \
+      --postgresql-port 5433 # <---- important \
+      [other options]
+
+... and use the random query generator in the same way:
+
+  $ tests/comparison/discrepancy_searcher.py \
+      --use-postgresql \
+      --postgresql-port 5433 # <---- important \
+      [other options]
+
+Testing
+-------
+
+The following steps have been taking to validate postgresql-9.5
+9.5.4-1.pgdg14.04+2 on Ubuntu 14.04.2 LTS:
+
+1. Migrating tpch and tpch_kudu from Impala into PostgresQL 9.5 (via
+  'data_generator.py migrate')
+2. Loading random data into PostgresQL 9.5 (via 'data_generator.py populate')
+3. Discrepancy tests between Impala and PostgresQL 9.5, and then again
+   on 9.3, to ensure results are the same.
+4. Quick experiments to make sure ON CONFLICT DO NOTHING and ON CONFLICT
+   DO UPDATE actually work when connecting to Postgres on port 5433.
+
+Other Resources
+---------------
+https://wiki.postgresql.org/wiki/Detailed_installation_guides
+https://wiki.postgresql.org/wiki/Apt/FAQ

tests/comparison/README

@@ -22,6 +22,10 @@ Requirements:
      sudo apt-get install python-mysqldb
      sudo apt-get install python-psycopg2   # Postgresql
 
+For Impala/Kudu CRUD random query generation and comparison, please see
+the supplemental POSTGRES.txt on setting up PostgresQL 9.5 or higher as
+a reference database.
+
 Please see the supplemental ORACLE.txt on setting up Oracle as a reference
 database.