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re2-prod-supervisor-heatwave-dualwrite

@ericallam ericallam tagged this 07 Aug 15:39
## Summary

Prisma expands `in` / `notIn` into one bind parameter per element, so
every distinct list
length is a separate prepared statement. Where the length tracks data
volume (a batch size,
a run-graph fan-out, a prior query's id set) one call site can mint
hundreds of them. Each
is used about once, but inserting it evicts an entry that was being
reused, so the cost
lands on unrelated queries sharing the pooler's statement cache. An
unbounded list also
risks the 65535 bind-parameter ceiling.

`boundedIn()` pads a filter list to the next power of two by repeating
its last element.
`IN` and `NOT IN` ignore duplicates, so results are unchanged, and a
call site drops from
one statement per length to at most `log2(cap)`. Applied to all existing
sites.

## Enforcement

Two oxlint rules require the helper: a list filter must be an inline
array literal or a
`boundedIn()` call.

- The first covers filters reached through `where` / `having` /
`cursor`, and deliberately
never descends into `data`, `create`, `update`, `set` or `equals`. A key
named `in` in
those positions is user data, not a predicate, and rewriting it would
corrupt what gets
  stored or compared.
- The second covers bare filter objects passed to where-building
helpers, which the first
cannot see. It found five sites in the run-graph batch loaders that were
otherwise
  invisible.

Both rules follow filters through the shapes they are actually written
in: conditional
expressions, logical-and objects, spread-conditional properties,
computed keys, and call
arguments. An array literal only counts as fixed-arity when nothing
spreads into it, since
`[...new Set(ids)]` has a runtime length. Twelve sites were hidden
behind those shapes
until the rules handled them.

Scoped to `in` and `notIn`. The scalar-list filters `hasSome` and
`hasEvery` compile to
`&& $1` and `@> $1`, passing the whole array as a single bind parameter,
so their arity never
reaches the statement text and there is nothing to bound.

Both rules are `error`, so new call sites fail CI. That ratchet has
already caught four
sites added by other PRs while this one was in review.

## Notes

`boundedIn` pads by repeating rather than with null: `x NOT IN (a, b,
NULL)` is never true,
so null-padding a `notIn` filter would silently return no rows. Lists
above 32768 are
returned unchanged so padding can never push a query past the parameter
limit.

Route modules reach the helper through `~/db.server` rather than
importing the database
barrel directly, since a value import of that barrel into a module that
also exports a React
component is only safe while dead-code elimination prunes it.

Measured on a local rig: 300 distinct list lengths produce 300 prepared
statements
unpadded, 10 padded. Verified end-to-end against a local stack with the
full task-suite
sweep, which surfaced no regressions.
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