TINYTEXT as a PRIMARY key in MySQL?

user_11541740 0
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Entries in my table are uniquely identified by word that is 5-10 characters long and I use TINYTEXT(10) for the column. However, when I try to set it as PRIMARY key I get the error that size is missing.

From my limited understanding of the docs, Size for PRIMARY keys can be used to simplify a way to detect unique value i.e. when first few character (specified by Size) can be enough to consider it unique match. In my case, the size would differ from 5 to 10 (they are all latin1 so they are exact byte per character + 1 for the lenght). Two questions:

  1. If i wanted to use TINYTEXT as PRIMARY key, which size should I specify? Maximum available - 10 in this case? Or should be the size strictly EXACT, for example, if my key is 6 character long word, but I specify Size for PK as 10 - it will try to read all 10 and will fail and throw me an exception?
  2. How bad performance-wise would be to use [TINY]TEXT for the PK? All Google results lead me to opinions and statements "it is BAD, you are fired", but is it really true in this case, considering TINYTEXT is 255 max and I already specified max length to 10?
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3 Answers

  1. MySQL/MariaDB can index only the first characters of the text fields but not the whole text if it is too large. The maximum key size is 3072 bytes and any text field larger than that cannot be used as KEY. Therefore on text fields longer than 3072 bytes you must specify explicitly how much characters it will index. When using VARCHAR or CHAR it can be done directly because you explicitly set it when declaring the datatype. It's not the case with *TEXT - they do not have that option. The solution is to create the primary key like this:

    CREATE TABLE mytbl (
    name TEXT NOT NULL,
    PRIMARY KEY idx_name(name(255))
);

    The same trick can be done if you need to make primary key on a VARCHAR field which is larger than 3072 bytes, on BINARY fields and BLOBs. Anyway you can imagine that if two large and different texts start with the same characters at the first 3072 bytes at the beginning, they will be treated as equal by the system. That may be a problem.

  2. It is generally bad idea to use text field as primary key. There are two reasons for that:

    2.1. It takes much more processing time than using integers to search in the table (WHERE, JOINS, etc). The link is old but still relevant;

    2.2. Any foreign key in another table must have the same datatype as the primary key. When you use text, this will waste disk space;

Note: the difference between *TEXT and VARCHAR is that the contents of the *TEXT fields are not stored inside the table but in outside memory location. Usually we do that when we need to store really large text.

user_136012210

for TINYTEXT can not specify the size. Use VARCHAR (size)

SQL Data Types

user_86768380

FYI, you can't specify a size for TINYTEXT in MySQL:

mysql> create table t1 ( t tinytext(10) );
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds 
to your MySQL server version for the right syntax to use near '(10) )' at line 1

You can specify a length after TEXT, but it doesn't work the way you think it does. It means it will choose one of the family of TEXT types, the smallest type that supports at least the length you requested. But once it does that, it does not limit the length of input. It still accepts any data up to the maximum length of the type it chose.

mysql> create table t1 ( t text(10) );
Query OK, 0 rows affected (0.02 sec)

mysql> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `t` tinytext
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

mysql> insert into t1 set t = repeat('a', 255);
Query OK, 1 row affected (0.01 sec)

mysql> select length(t) from t1;
+-----------+
| length(t) |
+-----------+
|       255 |
+-----------+
Bill_Karwin_208600
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