evazion 6dd331745a Rewrite related tags implementation. ...

Rewrite the implementation of related tags to be simpler, faster, and
more accurate:

* The related tags are now calculated by taking a random sample of 1000
  posts, finding the top 250 most frequent tags among those posts, then
  ordering those tags by cosine similarity.

* Related tags can generally be calculated in 50-300ms at these sample
  sizes. Very high sample sizes (25000+ posts) are still relatively fast
  (1-3 seconds), but generally they don't improve accuracy much.

* Related tags are now cached in redis rather than in the tags table.
  The related_tags column in the tags table is no longer used.

* Only the related tags in the search taglist are cached. The related
  tags returned by the 'Related tags' button are not cached.

* The cache lifetime is a fixed 4 hours.

* The 'Related tags' button now works with metatags.

* The /related_tag page now works with metatags and multitag searches.

Fixes #4134, #4146.

2019-08-30 20:03:36 -05:00

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