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Would a MySQL database run more efficiently with smaller varchar lengths?


I have a database with quite a few VARCHAR fields. When the database was first built the lengths of the columns were set a bit larger than absolutely necessary.

Now after, having used the DB for a while and run a lot of data through it, I have a better idea of how long the fields need to be and am wondering about if reducing the VARCHAR lengths would make it run better.

If I set the lengths to say 10 characters plus what is currently the max length would that help the select and join times?

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1 comment

The general answer to "Will X make Y faster?" is "Try it and measure" ‭klutt‭ about 1 month ago

2 answers


The manual writes:

In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.

That is, the only effect of specifying a shorter length limit is that it can enable the database to use 1 rather than 2 bytes to store the (byte) length of the text.

Specifically, this means that a VARCHAR(1) and a VARCHAR(63) are always stored and retrieved in the exact same way (assuming an database character encoding with at most 4 bytes per character such as utf8mb4). Depending on the character encoding used, a VARCHAR with a higher limit may require 1 additional byte to store the length.

That is, VARCHAR length limits have negligible performance impact, and it many cases have no performance impact at all.

The database provides VARCHAR length limit support not because the database needs a limit, but because an application might. For instance, if your user interface has room only for 20 characters, you may want to express this constraint in the database depending on your application architecture.

Historically, this was an important feature because many popular application programming languages such a COBOL used fixed length strings, and databases were often shared by many applications. Nowadays, applications usually handle overly long strings gracefully enough that such constraints are no longer needed (or at least, no longer needed at the database level).



A simple google search of VARCHAR size, showed that it is not an arbitrarily sized string, which means VARCHAR 150 and VARCHAR 2 would take up the same amount space. So, no-- I don't think there would be any speed gains (unless the cache is in play which would most likely have a performance impact). My advice to you is to go with the smaller size (if and only if your profiling efforts show that it is faster, otherwise it would be premature optimization).


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