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How much memory is allocated for a MySQL VARCHAR variable in a stored procedure?

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In a stored procedure let's say I have a VARCHAR variable,

DECLARE example_variable VARCHAR(100);

how much memory is allocated to it and is that independent of the length of the data in the variable?

The reason I ask is that I had a stored procedure that would run out of memory until I trimmed the VARCHAR lengths down. The actual data inside the variable never changed only the declared size.

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It's really puzzling that a low, fixed number of variables could ever exhaust your system memory. Was the stored procedure perhaps recursive? If you changed max_sp_recursion_depth, you might need to change thread_stack as well. Jirka Hanika‭ 8 days ago

Can you please post the relevant parts of your procedure? As written, this looks like an XY problem (i.e. the issue seems to be in the way the procedure is written not the VARCHAR length). Alexei‭ 8 days ago

Also, can you provide more information about "run out of memory"? Is it the stack (too much recursion?) or another type of memory? Alexei‭ 8 days ago

2 answers

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The example variable will use 101 bytes, independently of whether any data was already assigned to it.

This holds regardless of whether you are using InnoDB or MyISAM which otherwise differ in how they handle string storage.

An alternative fix to shortening the variables would be to change them to type text, or to increase thread_stack.

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This answers your questions, not what I suspect to be real issue. According to the specifications a VARCHAR(100) will need actual data stored length + 1.

So, the actual size would have mattered if you have used CHAR(100) instead:

The compact family of InnoDB row formats optimize storage for variable-length character sets. See COMPACT Row Format Storage Characteristics. Otherwise, M × w bytes, <= M <= 255, where w is the number of bytes required for the maximum-length character in the character set.

I am not sure what happens if the procedures manipulate strings longer than the VARCHAR columns allow it. If these are silently truncated, the execution might fit in the memory, but the end result is not what you are expecting.

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