Filter tags by prefix

[lemon24]

Getting only specific tags with get_tags() is useful when implementing a "class" of objects on top of tags.

Possible use cases:

• reactions (thumbs up/down etc., https://github.com/lemon24/reader/issues/228#issuecomment-817072794)
• comments / user notes (https://github.com/lemon24/reader/issues/228)
• categories (https://github.com/lemon24/reader/issues/184#issuecomment-689587006)
• not exposing reserved tags
  • subcase: a plugin getting only the tags pertaining to it (.plugin.myplugin.*)
• autocompletion

Open questions:

• Is prefix match common enough in underlying storages that we can bake it into the API? Likely yes.
• Idem for case insensitive matching.
• Should we support both including and excluding tags from the start?
• Should we also return / sort by the number of resources with that tag?

[lemon24]

Some notes on how to implement ~efficient case-insensitive prefix match on SQLite:

• Per Query Optimizer Overview # The LIKE Optimization, for the optimization to happen, "the column must indexed using built-in NOCASE collating sequence".
• Since we're adding an index anyway, we might also piggy-back the wildcard (global, any feed, any entry) on it; this should be useful for autocomplete ("give me all entry tags starting with xyz").
  • On further thought, this is needed only if we merge the 3 tag tables (global, feed, entry) into a single one.
• We need to either disallow % from the prefix (* too, just in case), or escape it with expr LIKE 'a\%b%' ESCAPE '\'.