Mysql – Optimised query to select document and possible annotations

join;MySQL

I have a database of small documents which users can view and annotate. Their annotations are not public. I'm looking to optimise a single query to read the document and annotations for the current user.

Originally I tried this:

SELECT content,select_start,select_end,select_colour
FROM documents
LEFT OUTER JOIN annotations
USING (document_id)
WHERE document_id=<some_document_id_here> AND (user_id=<some_user_id_here> or user_id IS NULL)

But it failed to return a document if another user had added annotations. So I'm now using this:

SELECT content,select_start,select_end,select_colour
FROM documents A
LEFT OUTER JOIN (
    SELECT document_id,select_start,select_end,select_colour
    FROM annotations
    WHERE user_id=<some_user_id_here>
) B
ON A.document_id = B.document_id
WHERE A.document_id=<some_document_id_here>

which works, but the inner select statement could create quite a sizeable temporary table. I'd like to know if there's a better way to achieve this.

Best Answer

The best I've been able to come up with is to limit the size of the temporary table as much as possible by changing the inner WHERE to:

WHERE user_id=<some_user_id_here> AND document_id=<some_document_id_here>

It means the document ID has to be repeated in the statement, but at least means that the inner table is small, and pretty much guaranteed to be in-memory.

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