PostgreSQL Query Cache - "pqc"

PostgreSQL Query Cache
“pqc”

March/1/2011

Satoshi Nagayasu
Uptime Technologies, LLC.

Copyright 2010-2011 Uptime Technologies, LLC. All rights reserved.

Overview
• By caching result responses from backend, a query cache daemon
(`pqcd’) makes query response faster “extremely” (x10~x100).
– Delegates queries in front of the backend, like a proxy.
– Waits connections on the different port (9999 by default).
– Intercepts and caches SELECT query results. (Query Cache)
– Manages lifecycle of the query cache.

Server side
Direct execution

PostgreSQL PostgreSQL
client backend(s)
Execution pqcd
with
the query
cache Cache Memory


Tech Specs
• Supports simple query executions, and prepared statements.
– Supports V3 protocol only. (V2 is not supported)

• Supports “Active cache mode”
– All SELECT query results would be stored in the query cache.
– User specified query would not be cached.

• Supports “Passive cache mode”
– All SELECT query results would NOT be cached.
– Only user specified SELECT queries would be cached.
– And/or queries which took longer execution time than the user-specified
duration, would be cached. (Not implemented yet)

• Supports cache invalidation (expiration or refreshing)
– By using the cache expiration timeout.
– By using “hint” with SELECT statements.
– By using “hint” to expire all query cache. (Not implemented yet)


The Query Cache Flow Overview
• After `pqcd’ receives a SELECT statement, if some result is
available in the query cache, `pqcd’ returns the result from the query
cache. If not available, it relays the query to the backend.

Receive a query
from frontend

Is cache No
available?
Yes
Not expired No Invalidate Forward the query
yet? the cache to backend
Yes
Fetch a result Retreive a result
from query cache from the backend

Store a result
Send to the frontend
in the query cache


Implementation
• Using KVS (Memcached) to hold the query cache.
– “libmemcached” as a memcached client library for C.
– A SQL statement as a key, and response messages as a value.
• `pqcd’, a query cache daemon, is implemented as a derivative of
pgpool.
– Un-used feature (code) removed, and several “hook” routines added.
Server side

PostgreSQL PostgreSQL
client pqcd backend(s)

Hook routines
libmemcached

Memcached


Deployment
• Required packages
– libevent 1.4.14b (required by memcached)
– memcached 1.4.5
– libmemcached 0.43

• How to install
– automake
– ./configure --prefix=$PREFIX
– make
– sudo make install
– cd $PREFIX/etc
– cp pqcd_hba.conf.sample pqcd_hba.conf

• Starting pqcd
– $PREFIX/bin/pqcd

• Stopping pqcd
– $PREFIX/bin/pqcd stop


Deployment (with RPMs)
• Required RPM
– uqc-libevent-1.4.14b-1.i386.rpm
– uqc-memcached-1.4.5-1.i386.rpm
– uqc-libmemcached-0.43-1.i386.rpm
– uqc-querycache-20110223-1.i386.rpm

• Configuration files
– /opt/uptime/querycache/etc/pqcd.conf
– /opt/uptime/querycache/etc/pqcd_hba.conf

• Starting pqcd
– /opt/uptime/querycache/bin/pqcd

• Stopping pqcd
– /opt/uptime/querycache/bin/pqcd stop


Configurations
• `pqcd.conf’ may be configured as you need. (in $PREFIX/etc)

• memcached_bin
– A path name to Memcached executable. (default is
“/opt/uptime/querycache/bin/memcached”)

• query_cache_mode
– The query cache mode. “active” or “passive”. (default is “active”)

• query_cache_expiration
– The timeout value for query cache expiration (in seconds, default is 30)


Cache hints
• Add as a comment at the beginning of SELECT statements
– /* cache:refresh */SELECT * FROM …
– <slash> <asterrisk> <space> <hint> <space> <asterisk> <slash>

• cache:on (default in Active cache mode)
– Looks at the cache first. If not found, then executes on the backend, and puts the
result into the cache.
• cache:off (default in Passive cache mode)
– Doesn’t look at the cache. Execute on the backend first, and doesn’t put the
result into the cache.
• cache:refresh
– Doesn’t look at the cache first. Executes on the backend, and puts the result into
the cache.
• cache:expire
– Invalidates query cache for specified statement. (Not implemented yet)
• cache:expireall
– Invalidates all query cache. (Not implemented yet)


Examples


Query execution and the cache

A regular scan
takes >400ms.

2nd execution completes
in 0.5ms with the cache.


A hint to handle the cache.

Add a hint to disable
the query cache.

Longer time.

Without a hint,
the query cache again.


Record updates and the cache

Delete all records.

Still an old value left
in the cache.


Refreshing the query cache

Add a hint to
refresh the cache.

The value in the
cache refreshed.

With hitting
the cache.


TODO
• To Do
– Handle a transaction state correctly on “Z (ready for query)” response.
– Make expandable cache values for >8192bytes.
– Make enable to handle cache key with >250 charactors. (limitation from
Memcached)
– Automatic query caching by watching duration time under the Passive cache
mode.
– Run benchmarks!

• Done
– PreparedStatement.
– Cache hints.
– Rename the binary to ‘pqcd’ in Makefile
– Start/stop Memcached from pqcd.
– Invalidation query cache by timeout. (query_cache_expiration option)
– Completes a pgbench run.
– Remove V2 protocol code. (prevent to be conneced with V2 at startup)
– `memcached_bin’ option added in pqcd.conf.
– Put a connected database name into the cache key.
– Remove un-used code (replication, master-slave, dual server under connection
pooling) – “Keep it simple, stupid!”
– Make it public as an opensource software!


Contact:
Uptime Technologies, LLC.
E-Mail: contact@uptime.jp
Web: http://www.uptime.jp/


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