Summary:
Ref T9979. This uses ngrams (specifically, trigrams) to build a reasonably efficient index for substring matching. Specifically, for a package like "Example", with ID 123, we store rows like this:
```
< ex, 123>
<exa, 123>
<xam, 123>
<amp, 123>
<mpl, 123>
<ple, 123>
<le , 123>
```
When the user searches for `exam`, we join this table for packages with tokens `exa` and `xam`. MySQL can do this a lot more efficiently than it can process a `LIKE "%exam%"` query against a huge table.
When the user searches for a one-letter or two-letter string, we only search the beginnings of words. This is probably what they want, the only thing we can do quickly, and a reasonable/expected behavior for typeaheads.
Test Plan:
- Ran storage upgrades and search indexer.
- Searched for stuff with "name contains".
- Used typehaead and got sensible results.
- Searched for `aabbccddeeffgghhiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz` and saw only 16 joins.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9979
Differential Revision: https://secure.phabricator.com/D14846
