MySQL Bugs: #81181: Unclear how queries work when a search term exceeds an index prefix

Bug #81181	Unclear how queries work when a search term exceeds an index prefix
Submitted:	22 Apr 2016 16:16	Modified:	4 Mar 2019 15:52
Reporter:	Eric Lewis	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Documentation	Severity:	S3 (Non-critical)
Version:	5.7	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
It is unclear in the Reference Manual how queries work when a search term exceeds an index prefix.

e.g. if a column has an index with a prefix length of 767 bytes/191 characters, what happens if a search term includes a 192 character string for that column?

As I've been told in the IRC support room and reading in Stack Overflow (http://stackoverflow.com/a/31529672/1104036) the query would use the index where it is useful to rule out rows that are definitely not a match, and then touch rows in the table that are possible matches. 

I think we could describe this behavior in the Reference Manual, as it is important to understand. 

How to repeat:
Look at the Reference Manual.

Suggested fix:
Perhaps describe this behavior here? on the How MySQL Uses Indexes page (http://dev.mysql.com/doc/refman/5.7/en/mysql-indexes.html)?

In principle, it is quite simple. Index will be used for all searches where the constant string is smaller or equal to the index prefix.

If this is not well explained , then it should be ......

Posted by developer:
 
The following section was updated:

https://dev.mysql.com/doc/refman/5.5/en/column-indexes.html#column-indexes-prefix

"If a search term exceeds the index prefix length, the index is used to exclude 
non-matching rows, and the remaining rows are examined for possible matches."