MySql SELECT Query performance issues in huge database - mysql

Question

I have a pretty huge MySQL database and having performance issues while selecting data. Let me first explain what I am doing in my project: I have a list of files. Every file should be analyzed with a number of tools. The result of the analysis is stored in a results table.
I have one table with files (samples). The table contains about 10 million rows. The schema looks like this:
idsample|sha256|path|...
The other (really small table) is a table which identifies the tool. Schema:
idtool|name
The third table is going to be the biggest one. The table contains all results of the tools I am using to analyze the files (The number of rows will be the number of files TIMES the number of tools). Schema:
id|idsample|idtool|result information| ...
What I am looking for is a query, which returns UNPROCESSED files for a given tool id (where no result exists yet).
The (most efficient) way I found so far to query those entries is following:
SELECT
s.idsample
FROM
samples AS s
WHERE
s.idsample NOT IN (
SELECT
idsample
FROM
results
WHERE
idtool = 1
)
LIMIT 100
The problem is that the query is getting slower and slower as the results table is growing.
Do you have any suggestions for improvements? One further problem is, that i cannot change the structure of the tables, as this a shared database which is also used by other projects. (I think) the only way for improvement is to find a more efficient select query.
Thank you very much,
Philipp

Answer 1

A left join may perform better, especially if idsample is indexed in both tables; in my experience, those kinds of "inquiries" are better served by JOINs rather than that kind of subquery.
SELECT s.idsample
FROM samples AS s
LEFT JOIN results AS r ON s.idsample = r.idsample AND r.idtool = 1
WHERE r.idsample IS NULL
LIMIT 100
;
Another more involved possible solution would be to create a fourth table with the full "unprocessed list", and then use triggers on the other three tables to maintain it; i.e.
when a new tool is added, add all the current files to that fourth table (with the new tool).
when a new file is added, add all the current tools to that fourth table (with the new file).
when a new result in entered, remove the corresponding record from the fourth table.