DB tests: Istanbul coverage adds ~50% wall time / ~38% CPU to the packages/db suite #2274

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1. jp-knj commented on Oct 28, 2026

   

   jp-knj

   on Oct 28, 2026

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   Follow-up: here is where the rest of the cost goes, beyond coverage.

   Method. I ran packages/db with --maxWorkers=4, changing one condition per run. Times are wall time and total CPU (user+sys). I measured on commit 26d38a4cd with Node 26.12.0, on an M3 Max. A repeated run of the same condition matched within 0.02 s. On Node 24.18.0 with origin/main + #2275 (300 files), the coverage cost has the same shape: 122 s / 421 s without coverage, 165 s / 522 s with it.

   Cost	Wall	CPU	Kind
   Istanbul coverage	+43–48 s	+100–140 s	Instrumentation
   tests/oracle-replay.test.ts	+20 s	+106 s	Process startup
   typecheck: { enabled: true }	+26 s	+35 s	tsc
   environment: 'jsdom' for all files	+15 s	+76 s	Environment setup (~75 of 285 files touch the DOM)
   tests/indexed-db/retirement-oracle.test.ts	+14 s	+8 s	Idle waiting, fixed in #2275 (64 s → 2.3 s)

   Notes:

   • oracle-replay.test.ts runs 38 tests. Each one calls spawnSync to start node --import tsx tests/oracle-replay.ts, which spawns vitest run. One replay costs about 1.7 s, and Vitest's own work is 0.55 s of that. The rest is process startup. The file runs serially in one worker for 77 s, which sets a floor on the suite's wall time.
   • Test time is concentrated. Before test(indexed-db): resume retirement oracle cuts at native events #2275, these two files accounted for 53% of summed per-file test time.
   • Coverage and the timeouts in Two @tanstack/db tests time out under CPU load because a cold module import counts against the 5 s default timeout #2238 are related but separate. Coverage made the cold in-body imports about 6× slower, and test(db): load cold module graphs outside the test timeout #2276 moves those imports out of the timed test body. Reducing coverage cost would hide that class of test defect, not fix it.

   Next I'm looking at reducing the replay test's process startup without weakening what it checks.