MySQL by default is vary lax with data validation. Silent conversions is a concept that is not a common practice in other databases. In MySQL, instead of throwing an error, a warning was thrown and many applications simply did not handle warnings. With the introduction of sql_mode=STRICT_ALL_TABLES (or TRADITIONAL), in MySQL 5, a better level of validation now exists.

My understanding was that Warnings are now thrown as Errors, therefore eliminating the need to do a SHOW WARNINGS to confirm any problems after every query (this is a performance overhead on a high volume system due to the round trip latency).

However I found an instance where MySQL in STRICT Mode still throws warnings, leading to the question, are there any other areas, and does the earlier statement “Warnings are now thrown as Errors” hold true.

Here is my seeding process to showing the problem.

mysql> create table i(i tinyint, unique key( i));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into i values(999);
Query OK, 1 rows affected (0.00 sec)

Using default settings, attempting to INSERT a duplicate row throws an error, using INSERT IGNORE does not.

mysql> insert into i values(999);
ERROR 1062 (23000): Duplicate entry '127' for key 'i'
mysql> insert ignore into i values(999);
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------+
| Level   | Code | Message                                    |
+---------+------+--------------------------------------------+
| Warning | 1264 | Out of range value for column 'i' at row 1 |
+---------+------+--------------------------------------------+
1 row in set (0.00 sec)

When using a Strict Mode, a recommendation for all new systems, it is generally accepted that warnings are translated into errors, which implies your could should never have to consider checking for warnings.

mysql> truncate table i;
mysql> set sql_mode=strict_all_tables;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into i values(999);
ERROR 1264 (22003): Out of range value for column 'i' at row 1
mysql> insert ignore into i values(999);
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------+
| Level   | Code | Message                                    |
+---------+------+--------------------------------------------+
| Warning | 1264 | Out of range value for column 'i' at row 1 |
+---------+------+--------------------------------------------+
1 row in set (0.00 sec)

mysql> set sql_mode=traditional;
Query OK, 0 rows affected (0.00 sec)

mysql> insert ignore into i values(9990);
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------+
| Level   | Code | Message                                    |
+---------+------+--------------------------------------------+
| Warning | 1264 | Out of range value for column 'i' at row 1 |
+---------+------+--------------------------------------------+
1 row in set (0.00 sec)

I should caveat this post also with using caution with INSERT IGNORE. This should only be used if you never care about errors which I would never consider as a best practice design approach.

I have been asked twice this week what version of MySQL I would choose for a new project.
As with most questions in life the answer is: It Depends?

In general I would now recommend for a new project to select 5.1, and he is why.

1. If it’s a new project and your not managing existing applications with older versions then 5.1 is slated for General Availability (GA) at some imminent time. Having been at Release Candidate (RC) for quite some time (almost 1 year), many people, both internally and in the community are just waiting for Sun/MySQL to get this version out.
2. MySQL 5.0 is in maintenance mode, it’s now 3 years old. MySQL is placing (I’m assuming) resourcing energies to current and future releases.
3. If your looking at releasing a product in the next 3 months for example, you do not want to consider the testing and deployment of a new version (e.g. 5.1) in the next 6-9 months.
4. Unless your comparing specific performance between 5.1 and 5.0 in your edge cases, for a new project start with 5.1 you should be testing and confirming performance and reliability here. The worse case is you can test in 5.0 of any specific problem.
5. 5.1 gives you new features of course, partitioning may be of benefit but don’t assume it’s going to be a great improvement unless you applications SQL naturally tended to the MySQL partitioning strengths.
6. The single biggest benefit is the Pluggable Storage Engine Architecture. This can give you some benefits, and in the case of transaction storage engines that are production ready, Innodb now has a pluggable version, much improved on the MySQL supplied version. There are a long list of other engines under development with relative strengths and weakness, however be wary of versions that require customized builds of MySQL.

There are some concerns where I don’t have answers? For example, if you have MySQL Support , is 5.1 supported? I know a common answer to problems in pre 5.0 versions is, have you tried upgrading to 5.1

Why is not released? This is good question, the answer is obviously a level of quality, however it is generally discussed that 5.1 is of better quality in existing features then 5.0. It is 5.1 specific features you need to be careful of. It’s important that you do read carefully the 5.1 Release Notes to see where bug fixes or compatibility changes are still occuring.

As with any choice in the Open Source world, some level of risk assessment is necessary. If you have good metrics and measurement in place for your system, and you adequately test your software, there is no reason not to now consider 5.1 as a viable alternative for new development.

As I post this I note, I see the yet unreleased 5.1.28 list of bugs still shows issues of concern.

I came across this error recently when trying to modify the data type of a column.

ERROR 1025 (HY000): Error on rename of './sakila/#sql-1d91_5' to './sakila/inventory' (errno: 150)

Not the first time, and not the last time. A common problem with InnoDB tables, is the lack of information, you need to dig deeper with the following command (and appropriate security a well organized security profile will NOT have).

mysql> SHOW ENGINE INNODB STATUS;

...

------------------------
LATEST FOREIGN KEY ERROR
------------------------
080717 20:00:28 Error in foreign key constraint of table sakila/inventory:
there is no index in the table which would contain
the columns as the first columns, or the data types in the
table do not match the ones in the referenced table
or one of the ON ... SET NULL columns is declared NOT NULL. Constraint:
,
  CONSTRAINT "fk_inventory_film" FOREIGN KEY ("film_id") REFERENCES "film" ("film_id") ON UPDATE CASCADE

...

You also need to dig though the output of the command to find this, on a larger system this can be quite a lot of information just to find the details of the error. (It would be nice if there was an easier way.)

The result of this error is the columns in a foreign key relationship need to be of the same data type. This is actually a good thing, and MySQL generally operates much better in joins when the join columns are consistent.

On the assumption that you use surrogate primary keys for tables, this is a candidate for a naming standard for primary keys should be the primary key name is unique column name within your schema.

For example, if you call all your primary key’s ‘id’, your foreign key’s are normally ‘table_id’. While this is a common approach that I promoted myself for many years, it’s easy to read and consistent, actually naming every primary key uniquely provides two great benefits.

First, you can easily identify relationships in your entire schema without even knowing about the schema in detail. Second, you can leverage the benefit of the INFORMATION_SCHEMA to in the case of this post, confirm the data types are consistent for all matching columns, even when Referential Integrity is not used.

So, instead of using ‘id’, use ‘actor_id’, ‘film_id’, ‘user_id’ etc. For any self join keys, I normally prefix with parent, so ‘parent_user_id’ for example if you have a hierarchy within a table.

I have seen on many engagements the column data type is defined as INT(1).

People have the misconception that this numeric integer data type is of the length of one digit, or one byte. (One digit is 0-9 an one byte is 0-255)

This is incorrect.

Integer

For integer numeric data types in MySQL, that is TINYINT, SMALLINT, MEDIUMINT, INT, BIGINT the (n) has no bearing on the size of data stored within the specific data type. The (n) is simply for display formatting.

In the MySQL Manual 10.2. Numeric Types you read This optional display width is used to display integer values having a width less than the width specified for the column by left-padding them with spaces. The display width does not constrain the range of values that can be stored in the column, nor the number of digits that are displayed for values having a width exceeding that specified for the column.

The following example shows the (n) in this case 3 has no effect on the size of data stored.

DROP TABLE IF EXISTS numeric_int;
CREATE TABLE numeric_int(i INT(3) NOT NULL);
INSERT INTO numeric_int VALUES (1),(22),(333),(444),(55555);
SELECT * FROM numeric_intG
i: 1
i: 22
i: 333
i: 444
i: 55555

Floating Point

When it comes to floating point precision of FLOAT and DOUBLE, the syntax of (m,n) has a different inteperation. The manual states A precision from 0 to 23 results in a four-byte single-precision FLOAT column. A precision from 24 to 53 results in an eight-byte double-precision DOUBLE column.
I will discuss this some more in a different post with some interesting findings.

And MySQL allows a non-standard syntax: FLOAT(M,D) or REAL(M,D) or DOUBLE PRECISION(M,D). Here, “(M,D)” means than values can be stored with up to M digits in total, of which D digits may be after the decimal point. For example, a column defined as FLOAT(7,4) will look like -999.9999 when displayed. MySQL performs rounding when storing values, so if you insert 999.00009 into a FLOAT(7,4) column, the approximate result is 999.0001.

So in the case of FLOAT,DOUBLE the (n) does both affect storage and presentation where it rounds the number as confirmed by the following test. Look a the last 2 rows for the rounding confirmation.

DROP TABLE IF EXISTS numeric_float;
CREATE TABLE numeric_float(f1 FLOAT(10,5)  NOT NULL);
INSERT INTO numeric_float values (1),(2.0),(3.12345),(4.123451),(5.123456);
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0
SELECT * FROM numeric_floatG
f1: 1.00000
f1: 2.00000
f1: 3.12345
f1: 4.12345
f1: 5.12346
5 rows in set (0.01 sec)

Fixed Precision

The DECIMAL data type (NUMBER is a synonym) stores numbers to a fixed number of precision. From the manual again When declaring a DECIMAL or NUMERIC column, the precision and scale can be (and usually is) specified; for example: salary DECIMAL(5,2)
In this example, 5 is the precision and 2 is the scale. The precision represents the number of significant digits that are stored for values, and the scale represents the number of digits that can be stored following the decimal point. If the scale is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

So in our test:

DROP TABLE IF EXISTS numeric_decimal;
CREATE TABLE numeric_decimal(f1 DECIMAL(10,5)  NOT NULL);
INSERT INTO numeric_decimal values (1),(2.0),(3.12345),(4.123451),(5.123456);
Query OK, 5 rows affected, 2 warnings (0.00 sec)
SELECT * FROM numeric_decimalG
f1: 1.00000
f1: 2.00000
f1: 3.12345
f1: 4.12345
f1: 5.12346

What is also interesting is that with a FLOAT, the rounding of a number greater then (n), produces no warnings, yet when using DECIMAL you will see warnings. These are:

INSERT INTO numeric_decimal values (1),(2.0),(3.12345),(4.123451),(5.123456);
Query OK, 5 rows affected, 2 warnings (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 2

mysql> show warnings;
+-------+------+-----------------------------------------+
| Level | Code | Message                                 |
+-------+------+-----------------------------------------+
| Note  | 1265 | Data truncated for column 'f1' at row 4 |
| Note  | 1265 | Data truncated for column 'f1' at row 5 |
+-------+------+-----------------------------------------+
2 rows in set (0.00 sec)

What is also interesting is that the manual states the following When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the allowable number of digits.)

The number is generally truncated, buy differs per OS. In the case on Mac O/S and Linux it is rounded. The two test environments in this case where:

mysql> show variables like '%version%';
+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| protocol_version        | 10                           |
| version                 | 5.1.23-rc                    |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | i686                         |
| version_compile_os      | apple-darwin9.0.0b5          |
+-------------------------+------------------------------+
5 rows in set (0.01 sec)

mysql> show variables like '%version%';
+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| protocol_version        | 10                           |
| version                 | 5.1.24-rc                    |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | i686                         |
| version_compile_os      | redhat-linux-gnu             |
+-------------------------+------------------------------+
5 rows in set (0.41 sec)

Conclusion

So just to conclude, (n) for Integer types is for display formatting only, (m,n) for floating point will round the number at n places, while in fixed point (m,n) n will round or truncate the number.

About the Author

Ronald Bradford provides Consulting and Advisory Services in Data Architecture, Performance and Scalability for MySQL Solutions. An IT industry professional for two decades with extensive database experience in MySQL, Oracle and Ingres his expertise covers data architecture, software development, migration, performance analysis and production system implementations. His knowledge from 10 years of consulting across many industry sectors, technologies and countries has provided unique insight into being able to provide solutions to problems. For more information Contact Ronald.

References

• 10.2. Numeric Types
• Top 20 Design Tips for Data Architects – Apr 2008