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Stored function behavior
Posted by: Michael Graffam
Date: August 16, 2019 11:55AM

Hello,

I have a table of data built from AWS data, with varchar columns like account, region, vpc, instance-id and so on. The code that builds this table takes the JSON representation of the tags, converts it to an XML representation, and puts that into a mediumtext column.

I have a stored function, GetTag which is defined as:
GetTag (xml mediumtext, foo varchar(128)) returns varchar(256)
return ExtractValue(xml,concat("/AWSTags/tag[Key='",foo,"']/Value"));

Just a simple helper function to make things cleaner and give a shorthand syntax.

I've found a difficulty, perhaps even a bug, in MySQL, that I've tracked down to the following example:

select count(awsid) from instances where awsid in
(select awsid from instances where GetTag(tags,'Name') like '%blah%');
This returns the full count of everything in `instances`

However, if I change the GetTag() call and inline the ExtractValue() representation, things work as expected and I get the count of instances with a tag name like 'blah'

Explanations? Help? Insights?

I'm really puzzled.

I am using Amazon Aurora RDS. I have not yet built an instance to try it on a real MySQL server; mostly because I'm thinking that I'm simply missing something subtle.

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