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--- mysql:error_1071 [2016/07/01 22:48] – peter
+++ mysql:error_1071 [2020/07/15 09:30] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== MySQL - Error #1071 - Specified key was too long; max key length is 767 bytes ======
-This error is received because 1 byte equals 1 character only if you use the **latin-1** character set.  If you use **utf8**, each character will be considered 3 bytes when defining your key column.  If you use **utf8mb4**, each character will be considered to be 4 bytes when defining your key column.  Thus, you need to multiply your key field's character limit by, 1, 3, or 4 (depending on which character set is being used) to determine the number of bytes the key field is trying to allow.  If you are using **uft8mb4**, you can only define a maximum of 191 characters for a native, InnoDB, primary key field. Just don't breach 767 bytes.
+The max key length depends on the character set you use.  If you use **latin1** then the largest column you can index is **varchar(767)** with 1 byte equalling 1 character.  If you use **utf8**, each character will be considered 3 bytes when defining your key column.  If you use **utf8mb4**, each character will be considered to be 4 bytes when defining your key column.
-When using **utf8mb4** the maximum number of characters is therefore 767/4≈191 characters.
+  * MySQL '**utf8**' encoding doesn't allow characters beyond U+FFFF.
+  * MySQL '**utf8mb4**' encoding allows characters beyond U+FFFF through U+FFFFFF.
-MySQL assumes worst case for the number of bytes per character in the string.  For the MySQL 'utf8' encoding, that's 3 bytes per character since that encoding doesn't allow characters beyond U+FFFF.  For the MySQL 'utf8mb4' encoding, it's 4 bytes per character, since that's what MySQL calls actual UTF-8.
+Thus, you need to multiply your key field's character limit by, 1, 3, or 4 (depending on which character set is being used) to determine the number of bytes the key field will actually try to use.
-Looking at an example:
+  * For **uft8** a maximum of 255 characters can therefore be used for an InnoDB key field because 767/3≈255.
+  * For **uft8mb4** a maximum of 191 characters can therefore be used for an InnoDB key field because 767/4≈191.
-Assuming that **utf8mb4** is being used, this means that 4 bytes are needed for every character.  Attempting to have a key column be 255 characters long will fail with this error because those 255 characters actually takes up 4 times that amount of bytes, i.e. 255 * 4 = 1020.  However as mentioned, the maximum length supported by InnoDB is 767, therefore you would need to change this from:
+There is also a separate 3072 byte limit per index.  The 767 byte limit is per column, so you can include multiple columns (each 767 bytes or smaller) up to 3072 total bytes per index, but no column longer than 767 bytes.  (MyISAM is a little different.  It has a 1000 byte index length limit, but no separate column length limit within that).
-<code mysql>
-ALTER TABLE `mytable` ADD UNIQUE ( column1(255));
-</code>
-to
+One workaround for these limits is to only index part of a field, for example, here only 191 characters from column1 is part of the index:
 <code mysql>
@@ Line 22: / Line 21: @@
+But what if you want to index more than 767 bytes of a column in InnoDB?
+In that case you should consider using **innodb_large_prefix**, which was introduced in MySQL 5.5.14 and allows you to include columns up to 3072 bytes long in InnoDB indexes.  It does not affect the index limit, which is still 3072 bytes as quoted in the manual:
-If you’ve ever tried to add an index that includes a long varchar column to an InnoDB table in MySQL, you may have seen this error:
 <code>
-ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
+The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes.
+This limit applies to the length of the combined index key in a multi-column index.
 </code>
-The character limit depends on the character set you use.  For example if you use **latin1** then the largest column you can index is **varchar(767)**, but if you use **utf8** then the limit is **varchar(255)**.  There is also a separate 3072 byte limit per index.  The 767 byte limit is per column, so you can include multiple columns (each 767 bytes or smaller) up to 3072 total bytes per index, but no column longer than 767 bytes. (MyISAM is a little different.  It has a 1000 byte index length limit, but no separate column length limit within that).
-One workaround for these limits is to only index a prefix of the longer columns, but what if you want to index more than 767 bytes of a column in InnoDB?
-In that case you should consider using **innodb_large_prefix**, which was introduced in MySQL 5.5.14 and allows you to include columns up to 3072 bytes long in InnoDB indexes.  It does not affect the index limit, which is still 3072 bytes as quoted in the manual:
-<code>
-The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes.  This limit applies to the length of the combined index key in a multi-column index.
-</code>
-Read on for details and examples about **innodb_large_prefix**.
+===== Using innodb_large_prefix =====
 Here are a few pre-requisites for using **innodb_large_prefix**:
@@ Line 62: / Line 40: @@
 The default file format is still Antelope for backwards compatibility, and the default row format is COMPACT.
-You can set both **innodb_file_format** and **innodb_large_prefix** dynamically, but you should also set them in my.cnf so they survive a restart.
+You can set both **innodb_file_format** and **innodb_large_prefix** dynamically, but you should also set them in **my.cnf** so they survive a restart.
 Here’s an example.  If I try to create this table with **innodb_large_prefix** disabled I get an error: