Speed up retrieval of lemma suggestions

[mbollmann]

Originally reported by: Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann)

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Users report that retrieving lemma suggestions can sometimes take up to 7 seconds, which is huge.

The testing script introduced in commit 1e1f888 suggests that the part which retrieves lemma suggestions from other texts within the project (with the same "ascii" token) is a major bottleneck, taking almost a full second for one lookup on the production database.

If there is no evidence for other factors, refactoring this lookup should be considered.

A possible solution could be to introduce another database table which maps (project_id, mod_ascii) pairs to tag_ids of lemma annotations. This table would have to be updated whenever lemma annotations are saved, and could then be used for efficient lookup during retrieval.

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• Bitbucket: https://bitbucket.org/mbollmann/cora/issue/69