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Displaying posts with tag: code (reset)
MySQL vs Drizzle plugin APIs

There’s a big difference in how plugins are treated in MySQL and how they are treated in Drizzle. The MySQL way has been to create a C API in front of the C++-like (I call it C- as it manages to take the worst of both worlds) internal “API”. The Drizzle way is to have plugins be first class citizens and use exactly the same API as if they were inside the server.

This means that MySQL attempts to maintain API stability. This isn’t something worth trying for. Any plugin that isn’t trivial quickly surpasses what is exposed via the C API and has to work around it, or, it’s a storage engine and instead you have this horrible mash of C and C++. The byproduct of this is that no core server features are being re-implemented as plugins. This means the API is being developed in a vacuum devoid of usefulness. At least, this was the case… The authentication plugin API seems to be an exception, and it’s …

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Some storage engine features you only get if you’re InnoDB

I had reason to look into the extended secondary index code in MariaDB and MySQL recently, and there was one bit that I really didn’t like.

MariaDB:

share->set_use_ext_keys_flag(legacy_db_type == DB_TYPE_INNODB);

MySQL:

use_extended_sk= (legacy_db_type == DB_TYPE_INNODB);

In case you were wondering what “legacy_db_type” actually does, let me tell you: it’s not legacy at all, it’s kind of key to how the whole “metadata” system in MySQL works. For example, to drop a table, this magic number is used to work out what storage engine to call to drop the table.

Now, these code snippets basically kiss goodbye to the idea of a “pluggable storage engine” architecture. If you’re not InnoDB, you don’t get to have certain features. This isn’t exactly MySQL or MariaDB encouraging an open storage engine ecosystem (quite the opposite really).

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The EXAMPLE storage engine

The Example storage engine is meant to serve mainly as a code example of the stub of a storage engine for example purposes only (or so the code comment at the start of ha_example.cc reads). In reality however, it’s not very useful. It likely was back in 2004 when it could be used as a starting point for starting some simple new engines (my guess would be that more than a few of the simpler engines started from ha_example.cc).

The sad reality is the complexity of the non-obviousness of the bits o the storage engine API you actually care about are documented in ha_ndbcluster.cc, ha_myisam.cc and ha_innodb.cc. If you’re doing something that isn’t already done by one of those three engines: good luck.

Whenever I looked at ha_example.cc I always wished there was something more behind it… basically hoping that InnoDB would get a better and cleaner API with the server and would use that rather than the layering violations it has to …

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The ARCHIVE Storage Engine

I wonder how much longer the ARCHIVE storage engine is going to ship with MySQL…. I think I’m the last person to actually fix a bug in it, and that was, well, a good number of years ago now. It was created to solve a simple problem: write once read hardly ever. Useful for logs and the like. A zlib stream of rows in a file.

You can actually easily beat ARCHIVE for INSERT speed with a non-indexed MyISAM table, and with things like TokuDB around you can probably get pretty close to compression while at the same time having these things known as “indexes”.

ARCHIVE for a long time held this niche though and was widely and quietly used (and likely still is). It has the great benefit of being fairly lightweight – it’s only about 2500 lines of code (1130 if you exclude azio.c, the slightly modified gzio.c from zlib).

It also use the table discovery mechanism that NDB uses. If you remove the FRM file for an ARCHIVE …

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The MySQL Cluster storage engine

This is one close to my heart. I’ve recently written on other storage engines: Where are they now: MySQL Storage EnginesThe MERGE storage engine: not dead, just resting…. or forgotten and The MEMORY storage engine. Today, it’s the turn of MySQL Cluster.

Like InnoDB, MySQL Cluster started outside of MySQL. Those of you paying attention at home may notice a correlation between storage engines not written exclusively for MySQL and being at all successful.

NDB (for Network DataBase) started inside Ericsson, originally written in a language called …

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TokuDB

Big news at Percona Live MySQL Conference and Expo this week, Tokutek open sourced TokuDB thus making my previous post Where are they now: MySQL Storage Engines out of date in just a few days.

In this case, I really don’t mind. It’s rather exciting that they’ve gone ahead and done this –  and it’s not just a code drop: https://github.com/Tokutek/ft-engine is where things are at, and recent commits were 2hrs and 18hrs ago which means that this is being maintained. This is certainly a good way to grow a developer community.

While being a MySQL engine is really interesting, the MongoDB integration is certainly something to watch – and may be …

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A few notes on InnoDB in MySQL 5.7.1

I’ve started poking around the MySQL 5.7.1 source tree (although just from tarball as I don’t see a BZR tree yet). I thought I’d share a few thoughts:

  • InnoDB temporary tables. Not REDO logged. What does this mean? It’s a huge step in removing the dependency on MEMORY and MyISAM engines for temporary tables used in query execution. With InnoDB temporary tables there is no reason for MEMORY engine to continue to exist, there is absolutely no way in which it is better.
  • InnoDB temp tables aren’t insert buffered
    This probably doesn’t really matter as you’re not going to be doing REDO logging for them (plus things are generally short lived)… but it could be a future area for performance improvement
  • The NO_REDO log mode appears to be implemented fairly neatly.
  • Improvements in innodb read only mode. What does this mean? Maybe we can finally get rid of the oddity of compressed …
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Where are they now: MySQL Storage Engines

There was once a big hooplah about the MySQL Storage Engine Architecture and how it was easy to just slot in some other method of storage instead of the provided ones. Over the years I’ve repeatedly mentioned how this wasn’t really

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30 configuration options and counting

While Domas may have rather effictively trolled the discussion with his post on howto configure table/user statistics (which gave me a good chuckle I do have to say), it’s at least incorrect for Percona Server as you have to enable the “userstat” server option :)

That being said, once enabled there are no extra configuration variables to think about. This is a huge advantage over configuring PERFORMANCE_SCHEMA - which has a total of THIRTY configuration options (31 if you include the global enable/disable option).

Some of these thirty odd configuration variables are only going to matter if you’re loading your own plugins, and even then, it’s probably only going to matter if they use the MySQL mutex implementations rather than, …

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Refactoring Internal temporary tables (another stab at it)

A few weekends ago, I started to again look at the code in Drizzle for producing internal temporary tables. Basically, we have a few type of tables:

  • Standard
  • Temporary (from CREATE TEMPORARY TABLE)
  • Temporary (from ALTER TABLE)
  • Internal temporary (to help with query execution)

If you’re lucky enough to be creating one of the first three types, you go through an increasingly lovely pile of code that constructs a nice protobuf message about what the table should look like and hands all responsibility over to the storage engine as to how to do that. The basic idea is that Drizzle gets the heck out of the way and lets the storage engine do its thing. This code path looks rather different than what we inherited from MySQL. For a start, we actually have a StorageEngine object rather than just lumping everything into the handler (which we correctly name a Cursor). However… the final …

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