Query optimizer fixes for the `who` and `stats` commands based on
a systematic performance audit of the SQLite query plans.
who command (expert/reviews/detail modes):
- Add INDEXED BY idx_notes_diffnote_path_created hints to all DiffNote
queries. SQLite's planner was selecting idx_notes_system (38% of rows)
over the far more selective partial index (9.3% of rows). Measured
50-133x speedup on expert queries, 26x on reviews queries.
- Reorder JOIN clauses in detail mode's MR-author sub-select to match
the index scan direction (notes -> discussions -> merge_requests).
stats command:
- Replace 12+ sequential COUNT(*) queries with conditional aggregates
(COALESCE + SUM + CASE). Documents, dirty_sources, pending_discussion_
fetches, and pending_dependent_fetches tables each scanned once instead
of 2-3 times. Measured 1.7x speedup (109ms -> 65ms warm cache).
- Switch FTS document count from COUNT(*) on the virtual table to
COUNT(*) on documents_fts_docsize shadow table (B-tree scan vs FTS5
virtual table overhead). Measured 19x speedup for that single query.
Database: 61652 docs, 282K notes, 211K discussions, 1.5GB.
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