Phase 7.4. Query joins

Field	Value
MEP	MEP-54
Status	LANDED
Started	2026-05-28 (GMT+7)
Landed	2026-05-28 (GMT+7)
Tracking PR	#22564
Commit	2ec49d122f

Gate

8 fixtures cover the join surface lifted from the JVM query_join/ baseline: join_cross_basic, join_cross_strings, join_cross_where, join_inner_int, join_inner_sum, join_inner_where, join_left_basic, join_left_filtered. Run: go test ./transpiler3/go/build/... -run TestPhase1Hello/join_.

Lowering decisions

The shared C lowerer's hash-join desugaring emits a map<K, list<K>> index keyed by the join key, plus a ForEach over the inner side that consults the index and skips when the key is absent. Phase 7.4 teaches the Go lowerer three new things to consume that shape:

1. lowerMapTypeWithList(K, list, E) -> map[K][]E. The new ListValueElemType field on aotir.LetStmt and aotir.MapLit carries the element type so the lowerer can produce the right Go map type. Without this, the C lowerer's map<int, list<int>> would lose the inner element type at the Go boundary.

2. StrConvertExpr for str(x) projections. Inner-int / float / bool conversions go through Go's strconv.Itoa(int(x)), strconv.FormatFloat(x, 'g', -1, 64), and strconv.FormatBool(x). The string -> string case is the identity. join_cross_strings exercises the projection path.

3. blockReferencesVar scan to drop unused ForEach iteration bindings. When the hash-join inner loop binds a row variable that the body never reads (the join collapses by key only), the Go output emits for range xs { ... } so Go's "declared and not used" check stays quiet.

The inner-join arm uses the index for O(1) probe per outer row; the left-join arm emits the same probe but with a if len(matches) == 0 { ... } branch to surface the unmatched outer row. The cross-join arm is the simple double-loop with no index. The RawExpr helper holds the inline type text (chan T, []T, etc.) so the new map[K][]E shape doesn't need a fresh AST node.

Files changed

File	Purpose
transpiler3/go/lower/types.go	lowerMapTypeWithList(K, value, listValueElem) -> map[K][]E when value==TypeList
transpiler3/go/lower/expr.go	StrConvertExpr (int/float/bool/string sources)
transpiler3/go/lower/stmt.go	blockReferencesVar scan -> drop unused ForEach bindings
tests/transpiler3/go/fixtures/join_*/	8 fixtures

Test set

• 8 TestPhase1Hello/join_* subtests covering inner / cross / left joins with int and string keys, with and without where post-filters.

Closeout notes

The blockReferencesVar scan was a Go-specific necessity: Go's compile-time "declared and not used" is the C runtime's silent dead store, so the lowerer had to learn to elide the binding. Adding the scan in 7.4 (rather than waiting for a Phase 7.7-time spurious test failure) saved an audit pass. The RawExpr-based map-type printing avoided adding a fresh MapTypeExpr AST node when the inline string was unambiguous.