Tag: InnoDB

Simple guideline for choosing appropriate InnoDB PRIMARY KEYs

MySQL

Risking some flames, I’d like to suggest only two options for choosing PRIMARY KEYs for InnoDB tables. I suggest they should cover 99% (throwing numbers around) of cases.

PRIMARY KEY cases

  1. An integer (SMALLINT / INT / BIGINT), possibly AUTO_INCREMENT column.
  2. The combination of two columns on a many-to-many connecting table (e.g. film_actor, which connects films to actors), the two columns being the PRIMARY KEYs of respective data tables. This rule may be extended to 3-way relation tables.

A short recap: an InnoDB must have a PRIMARY KEY. It will pick one if you don’t offer it. It can pick a really bad UNIQUE KEY (e.g. website_url(255)) or make one up using InnoDB internal row ids. If you don’t have a good candidate, an AUTO_INCREMENT PRIMARY KEY is probably the easiest way out.

A 2-column combination for a many-to-many connection table is common and viable. The PRIMARY KEY will not only provide with good join access method, but will also provide with the required UNIQUE constraint.

An integer-based PRIMARY KEY will make for more compact & shallow index tree structures, which leads to less I/O and page reads.

An AUTO_INCREMENT will allow for ascending PRIMARY KEY order of INSERT, which is InnoDB-friendly: index pages will be more utilized, less fragmented. Continue reading » “Simple guideline for choosing appropriate InnoDB PRIMARY KEYs”

Thoughts and ideas for Online Schema Change

Development MySQL

Here’s a few thoughts on current status and further possibilities for Facebook’s Online Schema Change (OSC) tool. I’ve had these thoughts for months now, pondering over improving oak-online-alter-table but haven’t got around to implement them nor even write them down. Better late than never.

The tool has some limitations. Some cannot be lifted, some could. Quoting from the announcement and looking at the code, I add a few comments. I conclude with a general opinion on the tool’s abilities.

“The original table must have PK. Otherwise an error is returned.”

This restriction could be lifted: it’s enough that the table has a UNIQUE KEY. My original oak-online-alter-table handled that particular case. As far as I see from their code, the Facebook code would work just as well with any unique key.

However, this restriction is of no real interest. As we’re mostly interested in InnoDB tables, and since any InnoDB table should have a PRIMARY KEY, we shouldn’t care too much.

“No foreign keys should exist. Otherwise an error is returned.”

Tricky stuff. With oak-online-alter-table, changes to the original table were immediately reflected in the ghost table. With InnoDB tables, that meant same transaction. And although I never got to update the text and code, there shouldn’t be a reason for not using child-side foreign keys (the child-side is the table on which the FK constraint is defined).

The Facebook patch works differently: it captures changes and writes them to a delta table,  to be later (asynchronously) analyzed and make for a replay of actions on the ghost table. Continue reading » “Thoughts and ideas for Online Schema Change”

How often should you use OPTIMIZE TABLE? – followup

MySQL

This post follows up on Baron’s How often should you use OPTIMIZE TABLE?. I had the opportunity of doing some massive purging of data from large tables, and was interested to see the impact of the OPTIMIZE operation on table’s indexes. I worked on some production data I was authorized to provide as example.

The use case

I’ll present a single use case here. The table at hand is a compressed InnoDB table used for logs. I’ve rewritten some column names for privacy:

mysql> show create table logs \G

Create Table: CREATE TABLE `logs` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(20) CHARACTER SET ascii COLLATE ascii_bin NOT NULL,
 `ts` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
 `origin` varchar(64) CHARACTER SET ascii COLLATE ascii_bin NOT NULL,
 `message` text NOT NULL,
 `level` tinyint(11) NOT NULL DEFAULT '0',
 `s` char(16) CHARACTER SET ascii COLLATE ascii_bin NOT NULL DEFAULT '',
 PRIMARY KEY (`id`),
 KEY `s` (`s`),
 KEY `name` (`name`,`ts`),
 KEY `origin` (`origin`,`ts`)
) ENGINE=InnoDB AUTO_INCREMENT=186878729 DEFAULT CHARSET=utf8 ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8

The table had log records starting 2010-08-23 and up till 2010-09-02 noon. Table status: Continue reading » “How often should you use OPTIMIZE TABLE? – followup”

Personal observation: more migrations from MyISAM to InnoDB

MySQL

I’m evidencing an increase in the planning, confidence & execution for MyISAM to InnoDB migration.

How much can a single consultant observe? I agree Oracle should not go to PR based on my experience. But I find that:

  • More companies are now familiar with InnoDB than there used to.
  • More companies are interested in migration to InnoDB than there used to.
  • More companies feel such migration to be safe.
  • More companies start up with an InnoDB based solution than with a MyISAM based solution.

This is the way I see it. No doubt, the Oracle/Sun deal made its impact. The fact that InnoDB is no longer a 3rd party; the fact Oracle invests in InnoDB and no other engine (Falcon is down, no real development on MyISAM); the fact InnoDB is to be the default engine: all these put companies at ease with migration.

Continue reading » “Personal observation: more migrations from MyISAM to InnoDB”

A MyISAM backup is blocking as read-only, including mysqldump backup

MySQL

Actually this is (almost) all I wanted to say. This is intentionally posted with all related keywords in title, in the hope that a related search on Google will result with this post on first page.

I’m just still encountering companies who use MyISAM as their storage engine and are unaware that their nightly backup actually blocks their application, basically rendering their product unavailable for long minutes to hours on a nightly basis.

So this is posted as a warning for those who were not aware of this fact.

There is no hot (non blocking) backup for MyISAM. Closest would be file system snapshot, but even this requires flushing of tables, which may take a while to complete. If you must have a hot backup, then either use replication – and take the risk of the slave not being in complete sync with the master – or use another storage engine, i.e. InnoDB.

Reducing locks by narrowing primary key

MySQL

In a period of two weeks, I had two cases with the exact same symptoms.

Database users were experiencing low responsiveness. DBAs were seeing locks occurring on seemingly normal tables. In particular, looking at Innotop, it seemed that INSERTs were causing the locks.

In both cases, tables were InnoDB. In both cases, there was a PRIMARY KEY on the combination of all 5 columns. And in both cases, there was no clear explanation as for why the PRIMARY KEY was chosen as such.

Choosing a proper PRIMARY KEY

Especially with InnoDB, which uses clustered index structure, the PRIMARY KEY is of particular importance. Besides the fact that a bloated PRIMARY KEY bloats the entire clustered index and secondary keys (see: The depth of an index: primer), it is also a source for locks. It’s true that any UNIQUE KEY can serve as a PRIMARY KEY. But not all such keys are good candidates. Continue reading » “Reducing locks by narrowing primary key”

Tip: faster than TRUNCATE

MySQL

TRUNCATE is usually a fast operation (much faster than DELETE FROM). But sometimes it just hangs; I’ve has several such uncheerful events with InnoDB (Plugin) tables which were extensively written to. The TRUNCATE hanged; nothing else would work; minutes pass.

TRUNCATE on tables with no FOREIGN KEYs should act fast: it translate to dropping the table and creating a new one (and it all depends on the MySQL version, see the manual).

What’s faster than TRUNCATE, then? If you don’t have triggers nor FOREIGN KEYs, a RENAME TABLE can come to the rescue. Instead of:

TRUNCATE log_table

Do:

CREATE TABLE log_table_new LIKE log_table;
RENAME TABLE log_table TO log_table_old, log_table_new TO log_table;
DROP TABLE log_table_old;

I found this to work well for me. Do note that AUTO_INCREMENT values can be tricky here: the “new” table is created with an AUTO_INCREMENT value which is immediately taken in the “working” table. If you care about not using same AUTO_INCREMENT values, you can: Continue reading » “Tip: faster than TRUNCATE”

Performance analysis with mycheckpoint

MySQL

mycheckpoint (see announcement) allows for both graph presentation and quick SQL access to monitored & analyzed data. I’d like to show the power of combining them both.

InnoDB performance

Taking a look at one of the most important InnoDB metrics: the read hit ratio (we could get the same graph by looking at the HTML report):

SELECT innodb_read_hit_percent FROM sv_report_chart_sample \G
*************************** 1. row ***************************
innodb_read_hit_percent: http://chart.apis.google.com/chart?cht=lc&chs=400x200&chts=303030,12&chtt=Nov+10,+11:40++-++Nov+11,+08:55+(0+days,+21+hours)&chdl=innodb_read_hit_percent&chdlp=b&chco=ff8c00&chd=s:400664366P6674y7176677677u467773y64ux166666764366646y616666666666644444434444s6u4S331444404433341334433646777666666074736777r1777767764776666F667777617777777777777777yaRi776776mlf667676xgx776766rou67767777u37797777x76676776u6A737464y67467761777666643u66446&chxt=x,y&chxr=1,99.60,100.00&chxl=0:||Nov+10,+15:55|Nov+10,+20:10|Nov+11,+00:25|Nov+11,+04:40|&chxs=0,505050,10

We see that read hit is usually high, but occasionally drops low, down to 99.7, or even 99.6. But it seems like most of the time we are above 99.95% read hit ratio. It’s hard to tell about 99.98%.

Can we know for sure?

We can stress our eyes, yet be certain of little. It’s best if we just query for the metrics! mycheckpoint provides with all data, accessible by simple SQL queries: Continue reading » “Performance analysis with mycheckpoint”

How to calculate a good InnoDB log file size – recap

MySQL

Following Baron Schwartz’ post: How to calculate a good InnoDB log file size, which shows how to make an estimate for the InnoDB log file size, and based on SQL: querying for status difference over time, I’ve written a query to run on MySQL 5.1, which, upon sampling 60 seconds of status, estimates the InnoDB transaction log bytes that are expected to be written in the period of 1 hour.

Recap: this information can be useful if you’re looking for a good innodb_log_file_size value, such that will not pose too much I/O (smaller values will make for more frequent flushes), not will make for a too long recovery time (larger values mean more transactions to recover upon crash).

It is assumed that the 60 seconds period represents an average system load, not some activity spike period. Edit the sleep time and factors as you will to sample longer or shorter periods. Continue reading » “How to calculate a good InnoDB log file size – recap”

InnoDB is dead. Long live InnoDB!

MySQL

I find myself converting more and more customers’ databases to InnoDB plugin. In one case, it was a last resort: disk space was running out, and plugin’s compression released 75% space; in another, a slow disk made for IO bottlenecks, and plugin’s improvements & compression alleviated the problem; in yet another, I used the above to fight replication lag on a stubborn slave.

In all those case, I needed to justify the move to “new technology”. The questions “Is it GA? Is it stable?” are being asked a lot. Well, just a few days ago the MySQL 5.1 distribution started shipping with InnoDB plugin 1.0.4. That gives some weight to the stability question when facing a doubtful customer.

But I realized that wasn’t the point.

Continue reading » “InnoDB is dead. Long live InnoDB!”