`connect()` used to run executescript(SCHEMA_SQL) + 7 forward-
migration probes on every open. Profile of `who wrote virt-back?`
on 38 GB of real shards (warm cache) showed 588 connect() calls
per query, each running the full probe sequence — 10,623 total
SQLite executes. Migrations are forward-only and idempotent within
a code version, so once we've run them on a path in this process
there's no work to do on subsequent opens.
Cache shape: `set[str]` keyed by `str(Path(p).resolve())`.
Migration block runs once per (path, process); subsequent calls on
the same shard skip it entirely. Per-connection PRAGMAs
(foreign_keys=ON, synchronous=NORMAL, cache_size, temp_store,
mmap_size) still run every time — SQLite scopes foreign_keys
per-connection and our schema's FK CASCADE behavior depends on it.
That's why `PRAGMA foreign_keys = ON` moved out of the cached
SCHEMA_SQL block into the always-run pragma section.
Cache invalidation: explicit only.
`store.invalidate_migration_cache(path)` for callers who replace a
shard at the same path (snapshot-restore flows). `_clear_migration_
cache()` for tests. We don't auto-detect file replacement —
(dev, inode) is unreliable under tmpfs inode reuse, and (mtime,
size) drifts naturally as SQLite operates on the file (WAL
checkpoints, page growth). Path-only with explicit invalidation
is the honest contract.
Re-profile (same query, same shards, warm cache):
metric before after
_migrate_* (each function) 586 7 ← per shard
executescript 586 7
SQLite executes 10,623 3,687 (-65%)
wall (warm) 14.5 s 13.4 s
The warm-cache wall delta is small because the probes were many-
but-cheap; residual cost lives in FTS5 search (6.7 s) and
synonym_expand (2.8 s, both separate concerns). The 65% execute
drop is the cold-cache win — each redundant executescript() had
been triggering disk reads at the 75 s scale the reviewer reported.
Tests (6, all green): first connect runs all 7 probes, second
connect runs zero, schema integrity preserved across re-opens,
explicit invalidation re-probes, distinct paths each get one probe,
clear-cache helper works.
Full suite: 1306 passed, 36 skipped. Found and fixed an FK CASCADE
regression mid-implementation: PRAGMA foreign_keys = ON was inside
SCHEMA_SQL, so memoization was silently turning it off on subsequent
opens. test_burn_doc.py caught it. Moved to the per-connection
pragma block.
Ticket #000026 status: Phase 1 landed; Phase 2 (baseline artifact)
and Phase 3 (warrant-quality finding) queued.
russell@unturf.com
ec92ebc575