diff --git a/src/aggregate.zig b/src/aggregate.zig
index 1f7c82d..3756c94 100644
--- a/src/aggregate.zig
+++ b/src/aggregate.zig
@@ -344,3 +344,161 @@ const AccState = struct {
         }
     }
 
+    /// Produces the final result `Value`, transferring ownership of any
+    /// heap payload (`.min`/`.max` on text/bytes fields) out of this
+    /// accumulator - a subsequent `deinit` sees `null` and frees nothing,
+    /// so the same payload is never freed twice. `.count_distinct` instead
+    /// frees its whole tracking set right here (the result is a plain
+    /// `.int`, nothing to hand ownership of), which is why this needs
+    /// `allocator` - both call sites (`GroupBy.finish`, `runGlobal`) skip a
+    /// separate `deinit` call after `take()` entirely, so this is the only
+    /// place that set is ever freed on the success path.
+    fn take(self: *AccState, allocator: Allocator) Value {
+        return switch (self.kind) {
+            .count => .{ .int = @intCast(self.n) },
+            .sum => if (self.allInt) .{ .int = self.sumInt } else .{ .float = self.sumFloat },
+            .avg => blk: {
+                if (self.n == 0) break :blk .{ .none = {} };
+                const total: f64 = if (self.allInt) @floatFromInt(self.sumInt) else self.sumFloat;
+                break :blk .{ .float = total / @as(f64, @floatFromInt(self.n)) };
+            },
+            .min => blk: {
+                const v = self.minv orelse Value{ .none = {} };
+                self.minv = null;
+                break :blk v;
+            },
+            .max => blk: {
+                const v = self.maxv orelse Value{ .none = {} };
+                self.maxv = null;
+                break :blk v;
+            },
+            .count_distinct => blk: {
+                const result = Value{ .int = @intCast(self.distinct.count()) };
+                var it = self.distinct.keyIterator();
+                while (it.next()) |k| allocator.free(k.*);
+                self.distinct.deinit(allocator);
+                self.distinct = .{};
+                break :blk result;
+            },
+        };
+    }
+
+    fn deinit(self: *AccState, allocator: Allocator) void {
+        if (self.minv) |v| v.free(allocator);
+        if (self.maxv) |v| v.free(allocator);
+        var it = self.distinct.keyIterator();
+        while (it.next()) |k| allocator.free(k.*);
+        self.distinct.deinit(allocator);
+    }
+};
+
+const Bundle = struct {
+    groupValue: Value,
+    accs: []AccState,
+
+    fn deinit(self: *Bundle, allocator: Allocator) void {
+        self.groupValue.free(allocator);
+        for (self.accs) |*a| a.deinit(allocator);
+        allocator.free(self.accs);
+    }
+};
+
+/// Streaming GROUP BY accumulator: call `observe` once per candidate row
+/// (already filtered by whatever `Predicate` the caller wants applied), then
+/// `finish` to materialize one `GroupRow` per distinct group-key value seen,
+/// in first-seen order (stable, not sorted - callers that want sorted
+/// output can sort the returned slice by `.group` using `row.compare`).
+pub const GroupBy = struct {
+    allocator: Allocator,
+    groupField: []const u8,
+    specs: []const AggSpec,
+    map: std.StringHashMapUnmanaged(*Bundle) = .{},
+    order: std.ArrayListUnmanaged([]const u8) = .{},
+
+    pub fn init(allocator: Allocator, groupField: []const u8, specs: []const AggSpec) GroupBy {
+        return .{ .allocator = allocator, .groupField = groupField, .specs = specs };
+    }
+
+    /// Safe to call after `finish()` (which empties the internal maps) or on
+    /// a `GroupBy` that was never `finish()`-ed (e.g. `observe` returned an
+    /// error partway through a batch) - either way, exactly what is still
+    /// owned gets freed once.
+    pub fn deinit(self: *GroupBy) void {
+        var it = self.map.iterator();
+        while (it.next()) |entry| {
+            self.allocator.free(entry.key_ptr.*);
+            entry.value_ptr.*.deinit(self.allocator);
+            self.allocator.destroy(entry.value_ptr.*);
+        }
+        self.map.deinit(self.allocator);
+        self.order.deinit(self.allocator);
+        self.map = .{};
+        self.order = .{};
+    }
+
+    /// Rows missing `groupField` entirely are excluded from every group
+    /// (no "null group" bucket) - the same omission `Table.createIndex`'s
+    /// backfill applies to rows missing an indexed field.
+    pub fn observe(self: *GroupBy, row: *const Row) AggregateError!void {
+        const groupVal = row.get(self.groupField) orelse return;
+        const key = try rowmod.encodeValue(self.allocator, groupVal);
+
+        if (self.map.get(key)) |bundle| {
+            self.allocator.free(key);
+            for (bundle.accs) |*a| try a.observe(self.allocator, row);
+            return;
+        }
+
+        // Scoped to this block so the errdefers below only guard the
+        // allocation itself: once `bundle` is handed to `self.map.put`
+        // just after, ownership has moved there, and a later failure (the
+        // final observe loop) must NOT free it out from under the map -
+        // an errdefer spanning the whole function would do exactly that
+        // and leave a dangling pointer in `self.map`.
+        const bundle = blk: {
+            const b = try self.allocator.create(Bundle);
+            errdefer self.allocator.destroy(b);
+            const accs = try self.allocator.alloc(AccState, self.specs.len);
+            errdefer self.allocator.free(accs);
+            for (self.specs, 0..) |spec, i| accs[i] = .{ .kind = spec.kind, .field = spec.field };
+            const dupedGroup = try groupVal.dupe(self.allocator);
+            b.* = .{ .groupValue = dupedGroup, .accs = accs };
+            break :blk b;
+        };
+
+        try self.map.put(self.allocator, key, bundle);
+        try self.order.append(self.allocator, key);
+        for (bundle.accs) |*a| try a.observe(self.allocator, row);
+    }
+
+    /// Consumes the accumulated groups into an owned `[]GroupRow` and
+    /// leaves `self` empty (a subsequent `deinit()` is then a no-op).
+    pub fn finish(self: *GroupBy) AggregateError![]GroupRow {
+        var results = try self.allocator.alloc(GroupRow, self.order.items.len);
+        var filled: usize = 0;
+        errdefer freeGroupRows(self.allocator, results[0..filled]);
+
+        for (self.order.items) |key| {
+            const bundle = self.map.get(key).?;
+            var values = try self.allocator.alloc(Value, bundle.accs.len);
+            errdefer self.allocator.free(values);
+            for (bundle.accs, 0..) |*a, j| values[j] = a.take(self.allocator);
+            results[filled] = .{ .group = bundle.groupValue, .values = values };
+            filled += 1;
+        }
+
+        var it = self.map.iterator();
+        while (it.next()) |entry| {
+            self.allocator.free(entry.key_ptr.*);
+            self.allocator.free(entry.value_ptr.*.accs);
+            self.allocator.destroy(entry.value_ptr.*);
+        }
+        self.map.deinit(self.allocator);
+        self.order.deinit(self.allocator);
+        self.map = .{};
+        self.order = .{};
+
+        return results;
+    }
+};
+