Message-ID: <5485ED3C.7020206@agliodbs.com>
Date: Mon, 08 Dec 2014 10:26:04 -0800
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
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To: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>,
 'Amit Kapila' <amit.kapila16@gmail.com>
CC: 'Robert Haas' <robertmhaas@gmail.com>,
 'Andres Freund' <andres@2ndquadrant.com>,
 'Alvaro Herrera' <alvherre@2ndquadrant.com>,
 'Bruce Momjian' <bruce@momjian.us>,
 'Pg Hackers' <pgsql-hackers@postgresql.org>
Subject: Re: On partitioning
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All,

Pardon me for jumping into this late.  In general, I like Alvaro's
approach.  However, I wanted to list the major shortcomings of the
existing replication system (based on complaints by PGX's users and on
IRC) and compare them to Alvaro's proposed implementation to make sure
that enough of them are addressed, and that the ones which aren't
addressed are not being addressed as a clear decision.  We can't address
*all* of the limitations of the current system, but let's make sure that
we're addressing enough of them to make implementing a 2nd partitioning
system worthwhile.

Where I have ? is because I'm not clear from Alvaro's proposal whether
they're addressed or not.

1.The Trigger Problem
the need to write triggers for INSERT/UPDATE/DELETE.
Addressed.

2. The Clutter Problem
cluttering up system views and dumps with hundreds of partitioned tables
Addressed.

3. Creation Problem
the need two write triggers and/or cron jobs to create new partitions
Addressed.

4. Creation Locking Problem
high probability of lock pile-ups whenever a new partition is created on
demand due to multiple backends trying to create the partition at the
same time.
Not Addressed?

5. Constant Problem
Since current partitioned query planning happens before the rewrite
phase, SELECTs do not use partition logic to evaluate even simple
expressions, let alone IMMUTABLE or STABLE functions.
Addressed??

6. Unique Index Problem
Cannot create a unique index across multiple partitions, which prevents
the partitioned table from being FK'd.
Not Addressed
(but could be addressed in the future)

7. JOIN Problem
Two partitioned tables being JOINed need to append and materialize
before the join, causing a very slow join under some circumstances, even
if both tables are partitioned on the same ranges.
Not Addressed?
(but could be addressed in the future)

8. COPY Problem
Cannot bulk-load into the Master, just into individual partitions.
Addressed.

9. Hibernate Problem
When using the trigger method, inserts into the master partition return
0, which Hibernate and some other ORMs regard as an insert failure.
Addressed.

10. Scaling Problem
Inheritance partitioning becomes prohibitively slow for the planner at
somewhere between 100 and 500 partitions depending on various factors.
No idea?

11. Hash Partitioning
Some users would prefer to partition into a fixed number of
hash-allocated partitions.
Not Addressed.

12. Extra Constraint Evaluation
Inheritance partitioning evaluates *all* constraints on the partitions,
whether they are part of the partitioning scheme or not.  This is way
expensive if those are, say, polygon comparisons.
Addressed.


Additionally, I believe that Alvaro's proposal will make the following
activities which are supported by partition-by-inheritance more
difficult or impossible.  Again, these are probably acceptable because
inheritance partitioning isn't going away.  However, we should
consciously decide that:

A. COPY/ETL then attach
In inheritance partitioning, you can easily build a partition outside
the master and then "attach" it, allowing for minimal disturbance of
concurrent users.  Could be addressed in the future.

B. Catchall Partition
Many partitioning schemes currently contain a "catchall" partition which
accepts rows outside of the range of the partitioning scheme, due to bad
input data.  Probably not handled on purpose; Alvaro is proposing that
we reject these instead, or create the partitions on demand, which is a
legitimate approach.

C. Asymmetric Partitioning / NULLs in partition column
This is the classic Active/Inactive By Month setup for partitions.
Could be addressed via special handling for NULL/infinity in the
partitioned column.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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