# Description of Changes
To resolve#3875, we added exact-match unique index point lookup support
to the C# bindings by introducing and using
`datastore_index_scan_point_bsatn`.
Previously, generated unique index `Find()` was (in at least one
codepath) implemented as:
* A range scan (`datastore_index_scan_range_bsatn`) over a BTree bound,
then
* `SingleOrDefault()` to collapse the results into a single row.
When the scan is empty, `SingleOrDefault()` returns `default(T)`. For
value-type rows this can manifest as a default-initialized row instead
of “missing”, which is what surfaced as “default-ish row” behavior in
views.
Using `datastore_index_scan_point_bsatn` makes the C# implementation
match Rust semantics more closely by performing an exact point lookup
and returning:
* `null` when no rows are found
* the row when exactly one row is found
* (defensively) an error if >1 row is returned (unique index invariant
violation)
Similarly, `datastore_delete_by_index_scan_point_bsatn` was added and
used so deletes-by-unique-key are also exact-match point operations
rather than range deletes.
Runtime updates were made to utilize point scan in `FindSingle(key)` and
in both mutable/read-only unique-index paths.
To keep this non-breaking for existing modules, codegen now detects
whether the table row is a struct or a class and chooses the appropriate
base type:
* Struct rows: `Find()` returns `Row?` (`Nullable<Row>`).
* Class rows: `Find()` returns `Row?` (nullable reference, `null` on
miss).
# API and ABI breaking changes
This change is non-breaking with respect to row type kinds, because
class/record table rows continue to work via
RefUniqueIndex/ReadOnlyRefUniqueIndex while struct rows use
UniqueIndex/ReadOnlyUniqueIndex.
API surface changes:
* Generated `Find()` return type is now nullable (`Row?`) to correctly
represent “missing”.
ABI/runtime:
* Requires the point-scan hostcall import
(`datastore_index_scan_point_bsatn`) to be available; the runtime uses
point-scan for unique lookup (and point delete for unique delete).
# Expected complexity level and risk
Low 2
# Testing
- [X] Local testing: repro module + client validate view and direct
Find() behavior
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Signed-off-by: rekhoff <r.ekhoff@clockworklabs.io>
Ryan