Add additional indexes to speed up report

[jwalits]

Whilst running the report, I found with around 4 million records in the local_csp table, the report would fall over at times.

Drilling down further into the error and sql query - found this one was causing some grief
https://github.com/catalyst/moodle-local_csp/blob/master/classes/table/csp_report.php#L178-L179

It would be good to add an index for both violateddirective and blockeddomain columns to speed up the query

Tested locally, Postgres was fine and was able to add index to those fields. However, when trying to add an index to those fields in MySQL came across this error
Specified key was too long; max key length is 3072 bytes.

After a chat with Brendan, a thought was converting the column to text to add an index.

I need to re-import the data again locally from prod and will test converting the columns to text and adding an index. Will create a PR with that shortly.

[brendanheywood]

Or the other option is adding a derived column which has a hash or similar in it, and it is indexed and we adjust the sql so we are leveraging the derived column instead. Which is almost like a poor mans index in the table.

[danmarsden]

Surely we don't need to keep 4 million records of csp report data? Isn't that a sign we aren't cleaning up?

[jwalits]

That was caused by a previous bug in the plugin (I believe it was this one: #59) The data has since been removed on the client site and rebuilding it currently, but with those many rows the report stopped execution.