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From the mysql docs,

1 - on joins,

The maximum number of tables that can be referenced in a single join is 61. This also applies to the number of tables that can be referenced in the definition of a view.

2 - row size

Every table (regardless of storage engine) has a maximum row size of 65,535 bytes

3 - on number of columns

There is a hard limit of 4096 columns per table

InnoDB permits up to 1000 columns.]

Is there an explanation of how these numbers have been calculated, or are these all constants assumed during system design?

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  1. The 61 is probably arbitrary. If you have a 61-way JOIN, you have an awful schema.

  2. The row size limit is not quite right. What page is that on? It is ignoring that MEDIUM/LONGTEXT/BLOB fields are not included in that limit. (But, for that matter, I don't know where 64K-1 applies.) In InnoDB, there is an "about 8K" limit excluding TEXT, etc that can be pushed to off-block storage.

  3. The InnoDB limit was raised to 1017 recently. The 4096 limit smells like a 12-bit fields in the Handler (Engine-agnostic) code.

I like limits, too. Here are some more.

  • That website is fantastic, and thanks for your answer :) – mu 無 Aug 22 '15 at 8:00
  • I am going through other posts on your site, and they are really informative. I've been looking for a similar resource for a few days now to get a better understanding of mysql internals. I will offer a bounty for your answer once the link is available to me. Thanks again :) – mu 無 Aug 22 '15 at 8:11
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    Thanks. My posts were derived from interacting with hundreds of DBAs, engineers, architects, plus from thousands of forum questions. I see a recurring question/problem, I think about it, study it, experiment, then I post a focused solution. The charset blog needs a major overhaul; I'm not happy with it. I started worrying about that topic a decade ago. I'll bet your handle gets Mojibaked into mu ç„¡. – Rick James Aug 22 '15 at 16:41

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