Michael Paquier

This week, I spent a long time working on this commit. Just by looking at the date, commit happened before leaving for week-end
commit 8ef0c48acadec3c9888d302888a7d279d82323e5
Author: Michael P
Date: Fri Jan 13 16:05:00 2012 +0900
　
Improve target list selection for remote DML queries
　
This commit makes remote DML planning generally available
for replicated and hash tables. There are still issues
related to node selection for round robin tables though.
The target list of UPDATE and DELETE using coordinator quals
was set to fetch only CTID when generating SELECT in their
inner plan generated by create_remotequery_plan.
　
Their target list is rewritten to include the columns in quals
so as to be able to evaluate those quals correctly on Coordinator.
In addition remote planning for UPDATE has been improved to be
able to target correct node when launching query.
　
A new regression test called xc_remote is added, it uses the
parameter enable_fast_query_shipping to force all the queries
to go through standard planner. Tests are done on replicated,
hash and round robin tables.

In all the examples of this article, those two tables are used with the following cluster configuration of Postgres-XC cluster.
db=# select node_name, node_type from pgxc_node; -- 1 Coordinator, 2 Datanodes
node_name | node_type
-----------+-----------
coord1 | C
dn1 | D
dn2 | D
(3 rows)
db=# create table aa (a int, b timestamp) distribute by hash(a);
CREATE TABLE
db=# create table bb (a int, b timestamp) distribute by replication;
CREATE TABLE


For database clusters in general, it is essential to have an efficient and consistent way to manage queries on both local and remote nodes. Efficiency is important to reduce data load on the system. Consistency is even more important to avoid dirty data in your database. So, about queries in general, let’s use an example. SELECT queries may contain expressions that can be evaluated on remote nodes. A common example for that is when the expression is a constant.
db=# explain verbose select * from aa where a = 1;
QUERY PLAN
---------------------------------------------------
Data Node Scan (cost=0.00..0.00 rows=0 width=0)
　　Output: a, b
　　Node/s: dn1
　　Remote query: SELECT a, b FROM aa WHERE (a = 1)
(4 rows)

In this case the query can be completely shipped to the remote node, returning correct results.

Expressions that cannot be pushed down are those who need to be evaluated on local nodes with all the necessary data fetched from remote nodes. For example, let’s take the replicated table bb. We want to select data on it with a time-based expression. Each node of the cluster (at least in the case of Postgres-XC) is located on a different server, each server having a different time line.So, is the following SQL shippable?
SELECT a from bb where b < now();
The answer is no. What is necessary to do is to get all the tuples (a,b) from table bb (a is necessary to send back result), and then apply the time based condition on all the results (explaining why b is necessary).
This results in the following plan.
db=# explain verbose select a from bb where b < now();
QUERY PLAN
-----------------------------------------------------------------
　Result (cost=0.00..1.01 rows=1000 width=4)
　Output: a
　-> Data Node Scan on bb (cost=0.00..1.01 rows=1000 width=4)
　　　Output: a, b
　　　Node/s: dn1
　　　Remote query: SELECT a, b FROM ONLY bb WHERE true
　　　Coordinator quals: (bb.b < now())
(7 rows)


Well, Postgres-XC has already a lot of mechanisms to manage SELECT and INSERT queries. But what was missing are the parts related to UPDATE and DELETE. So the new functionality committed this week allows to use complex expressions.
For example, in the case of update, you can run sequence and time based updates needing local node evaluation to run consistently.
db=# insert into bb values (1,now());
INSERT 0 1
db=# insert into bb values (2,now());
INSERT 0 1
db=# insert into bb values (3,now());
INSERT 0 1
db=# select * from bb;
a | b
---+---------------------------------
1 | Fri Jan 13 06:26:32.872665 2012
2 | Fri Jan 13 06:26:38.261489 2012
3 | Fri Jan 13 06:26:40.943182 2012
(3 rows)
db=# update bb set a = nextval('seq'), b = now();
UPDATE 3
db=# select * from bb;
a | b
---+---------------------------------
1 | Fri Jan 13 06:28:01.273496 2012
2 | Fri Jan 13 06:28:01.273496 2012
3 | Fri Jan 13 06:28:01.273496 2012
(3 rows)
db=# explain verbose update bb set a = nextval('seq'), b = now();
QUERY PLAN
-----------------------------------------------------------------------
　Update on public.bb (cost=0.00..11.01 rows=1000 width=6)
　Node/s: dn1, dn2
　Remote query: UPDATE public.bb SET a = $1, b = $2 WHERE ctid = $3
　-> Result (cost=0.00..11.01 rows=1000 width=6)
　　　Output: nextval('seq'::regclass), now(), ctid
　　　-> Data Node Scan on bb (cost=0.00..1.01 rows=1000 width=6)
　　　　　　Output: ctid
　　　　　　Node/s: dn1
　　　　　　Remote query: SELECT ctid FROM ONLY bb WHERE true
(9 rows)

You need here to select all the data to be updated from remote nodes, then you have to apply the time base expression (now) and the sequence value (nextval), and finally push those values to dedicated remote nodes.

This works also with WHERE clauses using non-shippable expressions.
db=# explain verbose update bb set a = nextval('seq'), b = now() WHERE b < now();
QUERY PLAN
---------------------------------------------------------------------------------
　Update on public.bb (cost=0.00..11.02 rows=1000 width=14)
　　Node/s: dn1, dn2
　　Remote query: UPDATE public.bb SET a = $1, b = $2 WHERE b = $3 AND ctid = $4
　　-> Result (cost=0.00..11.02 rows=1000 width=14)
　　　　Output: nextval('seq'::regclass), now(), b, ctid
　　　　-> Data Node Scan on bb (cost=0.00..1.01 rows=1000 width=14)
　　　　　　Output: b, ctid
　　　　　　Node/s: dn1
　　　　　　Remote query: SELECT b, ctid FROM ONLY bb WHERE true
　　　　　　Coordinator quals: (bb.b < now())
(10 rows)

Here what is added is a condition to pre-select a subset of rows. Such operation is costly though because you have to fetch all the rows of the table first in inner plan.

The same kind of crazy SQL are also possible for DELETE with mixing shippable and non-shippable expressions.
db=# explain verbose delete from bb where a = 2 and b < now();
QUERY PLAN
------------------------------------------------------------------------------
　Delete on public.bb (cost=0.00..1.02 rows=1000 width=18)
　Node/s: dn1, dn2
　Remote query: DELETE FROM public.bb WHERE a = $1 AND b = $2 AND ctid = $3
　-> Result (cost=0.00..1.02 rows=1000 width=18)
　　　Output: a, b, ctid
　　　-> Data Node Scan on bb (cost=0.00..1.02 rows=1000 width=18)
　　　　　Output: a, b, ctid
　　　　　Node/s: dn1
　　　　　Remote query: SELECT a, b, ctid FROM ONLY bb WHERE (a = 2)
　　　　　Coordinator quals: (bb.b < now())
(10 rows)

You can notice here that the constant expression "a = 2" is shipped in the most inner plan, improving query efficiency by that much.
A lot of things are now possible, and all this stuff will be included in release 0.9.7!