The plan for FDW-based sharding

There was discussion at the FOSDEM/PGDay Developer Meeting
(https://wiki.postgresql.org/wiki/FOSDEM/PGDay_2016_Developer_Meeting)
about sharding so I wanted to outline where I think we are going with
sharding and FDWs.

First, let me point out that, unlike pg_upgrade and the Windows port,
which either worked or didn't work, sharding is going be implemented and
useful in stages. It will take several years to complete, similar to
parallelism, streaming replication, and logical replication.

Second, as part of this staged implementation, there are several use
cases that will be shardable at first, and then only later, more complex
ones. For example, here are some use cases and the technology they
require:

1. Cross-node read-only queries on read-only shards using aggregate
queries, e.g. data warehouse:

This is the simplest to implement as it doesn't require a global
transaction manager, global snapshot manager, and the number of rows
returned from the shards is minimal because of the aggregates.

2. Cross-node read-only queries on read-only shards using non-aggregate
queries:

This will stress the coordinator to collect and process many returned
rows, and will show how well the FDW transfer mechanism scales.

3. Cross-node read-only queries on read/write shards:

This will require a global snapshot manager to make sure the shards
return consistent data.

4. Cross-node read-write queries:

This will require a global snapshot manager and global snapshot manager.

In 9.6, we will have FDW join and sort pushdown
(http://thombrown.blogspot.com/2016/02/postgresql-96-part-1-horizontal-s
calability.html). Unfortunately I don't think we will have aggregate
pushdown, so we can't test #1, but we might be able to test #2, even in
9.5. Also, we might have better partitioning syntax in 9.6.

We need things like parallel partition access and replicated lookup
tables for more join pushdown.

In a way, because these enhancements are useful independent of sharding,
we have not tested to see how well an FDW sharding setup will work and
for which workloads.

We know Postgres XC/XL works, and scales, but we also know they require
too many code changes to be merged into Postgres (at least based on
previous discussions). The FDW sharding approach is to enhance the
existing features of Postgres to allow as much sharding as possible.

Once that is done, we can see what workloads it covers and
decide if we are willing to copy the volume of code necessary
to implement all supported Postgres XC or XL workloads.
(The Postgres XL license now matches the Postgres license,
http://www.postgres-xl.org/2015/07/license-change-and-9-5-merge/.
Postgres XC has always used the Postgres license.)

If we are not willing to add code for the missing Postgres XC/XL
features, Postgres XC/XL will probably remain a separate fork of
Postgres. I don't think anyone knows the answer to this question, and I
don't know how to find the answer except to keep going with our current
FDW sharding approach.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
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