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Displaying posts with tag: data types (reset)
But I DO want MySQL to say “ERROR”!

MySQL is known for its willingness to accept invalid queries, data values. It can silently commit your transaction, truncate your data.

  • Using GROUP_CONCAT with a small group_concat_max_len setting? Your result will be silently truncated (make sure to check the warnings though).
  • Calling CREATE TEMPORARY TABLE? You get silent commit.
  • Issuing a ROLLBACK on non-transactional involved engines? Have a warning; no error.
  • Using LOCK IN SHARE MODE on non transactional tables? Not a problem. Nothing reported.
  • Adding a FOREIGN KEY on a MyISAM table? Good for you; no action actually taken.
  • Inserting 300 to a TINYINT column in a relaxed sql_mode? …
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Useful temporal functions & queries

Here’s a complication of some common and useful time & date calculations and equations. Some, though very simple, are often misunderstood, leading to inefficient or incorrect implementations.

There are many ways to solve such problems. I’ll present my favorites.

Querying for time difference

Given two timestamps: ts1 (older) and ts2 (newer), how much time has passed between them?

One can use TIMEDIFF() & DATEDIFF(), or compare two UNIX_TIMESTAMP() values. My personal favorite is to use TIMESTAMPDIFF(). Reason being that I’m usually interested in a specific metric, like the number of hours which have passed, or the number of days, disregarding the smaller minute/second resolution. Which allows one to:

SELECT TIMESTAMPDIFF(HOUR, ts1, ts2)

Take, for example:

SELECT TIMESTAMPDIFF(MONTH, …
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Character sets: latin1 vs. ascii

Unless specified otherwise, latin1 is the default character set in MySQL.

What I usually find in schemes are columns which are either utf8 or latin1. The utf8 columns being those which need to contain multilingual characters (user names, addresses, articles etc.), and latin1 column being all the rest (passwords, digests, email addresses, hard-coded values etc.)

I find latin1 to be improper for such purposes and suggest that ascii be used instead. The reason being that latin1 implies a European text (with swedish collation). It is unclear for an outsider, when finding a latin1 column, whether it should actually contain West European characters, or is it just being used for ascii text, utilizing the fact that a character in latin1 only requires 1 byte of storage. …

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The depth of an index: primer

InnoDB and MyISAM use B+ and B trees for indexes (InnoDB also has internal hash index).

In both these structures, the depth of the index is an important factor. When looking for an indexed row, a search is made on the index, from root to leaves.

Assuming the index is not in memory, the depth of the index represents the minimal cost (in I/O operation) for an index based lookup. Of course, most of the time we expect large portions of the indexes to be cached in memory. Even so, the depth of the index is an important factor. The deeper the index is, the worse it performs: there are simply more lookups on index nodes.

What affects the depth of an index?

There are quite a few structural issues, but it boils down to two important factors:

  1. The number of rows in the table: obviously, more rows leads to larger index, larger indexes grow in depth.
  2. The size of the indexed column(s). An index on an …
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MySQL’s character sets and collations demystified

MySQL’s character sets and collations are often considered as a mystery, and many users either completely disregard them and keep with the defaults, or set everything to UTF8.

This post will attempt to shed some light on the mystery, and provide with some best practices for use with text columns with regard to character sets.

Character Sets

A thorough discussion of how the character sets have evolved through history is beyond the scope of this post. While the Unicode standard is gaining recognition, the “older” character sets are still around. Understanding the difference between Unicode and local character sets is crucial.

Consider, for example, MySQL’s latin1 character set. In this character set there are 256 different characters, represented by one byte. The first 128 characters map to ASCII, the standard “ABCabc012 dot comma” set, of which most of this post is …

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Common wrong Data Types compilation

During my work with companies using MySQL, I have encountered many issues with regard to schema design, normalization and indexing. Of the most common errors are incorrect data types definition. Many times the database is designed by programmers or otherwise non-expert DBAs. Some companies do not have the time and cannot spare the effort of redesigning and refactoring their databases, and eventually face poor performance issues.

Here’s a compilation of “the right and the wrong” data types.

  • INT(1) is not one byte long. INT(10) is no bigger than INT(2). The number in parenthesis is misleading, and only describes the text alignment of the number, when displayed in an interactive shell. All mentioned types are the same INT, have the same storage capacity, and the same range. If you want a one-byte …
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MySQL INT(1) or INT(10)

In short, it really doesn’t matter.  After watching a MySQL web conference by Jay Pipes, I was gutted when I found out that they are actually exactly the same.  I know im not alone in thinking that it affected the size of the data field.  An unsigned int has the max value of 4294967295 no matter if its INT(1) or int(10) and will use 4 bytes of data.

So, what does the number in the brackets mean?  It pretty much comes down to display, its called the display-width.  The display width is a number from 1 to 255. You can set the display width if you want all of your integer values to “appear”.  If you enable zerofill on the row, the field will have a default value of 0 for int(1) and 0000000000 for int(10).

There are 5 main numeric data types, and you should …

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How to emulate the TYPEOF() function in MySQL

Want to know the type of an arbitrary expression in MySQL? Someday in the far far future in version 7.1, you might be able to with the TYPEOF() function.

For now you can try this:

CREATE TEMPORARY TABLE typeof AS SELECT [expression] AS col;

For example, let’s see what the type of CRC32 is.

mysql> CREATE TEMPORARY TABLE typeof AS SELECT CRC32('hello world') AS col;
mysql> DESCRIBE typeof;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| col   | int(10) unsigned | NO   |     | 0       |       | 
+-------+------------------+------+-----+---------+-------+

This is one possible way to programmatically determine the type of an expression — even an arbitrarily complex one.

Not beautiful, …

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DB Basics ? February 2008 Boston MySQL User Group Meeting

Here is the video of “Database Basics”, which I presented at the February 2008 Boston MySQL User Group meeting. The presentation goes over the basics of relations, data, the Entity-Relationship Model, how to choose data types, and how to do basic CREATE statements.

You can download:

the video (Large, 500 MB, or Small, 100 MB)
and
the slides (PDF, 171 Kb).

Or just watch the video:

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