diff --git a/src/query.zig b/src/query.zig
index fc98432..979ec5f 100644
--- a/src/query.zig
+++ b/src/query.zig
@@ -2381,3 +2381,159 @@ test "explain reports composite_eq and composite_range with matched/total field
     var db = try openTemp(&tmp, allocator);
     defer db.close();
 
+    var table = try Table.open(&db, allocator, "users");
+    defer table.deinit();
+    try table.createCompositeIndex("city_age", &.{ "city", "age" });
+    try table.put(.{ .int = 1 }, &.{
+        .{ .name = "city", .value = .{ .text = "warsaw" } },
+        .{ .name = "age", .value = .{ .int = 30 } },
+    });
+
+    // Only the leading column pinned -> composite_eq over a strict prefix
+    // (matchedFields 1 of 2). `findCompositeMatch` only looks inside a
+    // top-level `and_` (see its doc comment), so a bare single `eq` outside
+    // one does not reach the composite index at all - wrapping in `and_`
+    // with itself as the sole conjunct is what actually exercises the
+    // leftmost-prefix match here.
+    const prefixPreds = [_]Predicate{
+        .{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } },
+    };
+    var prefixOnly = try table.explain(allocator, .{ .and_ = &prefixPreds });
+    defer prefixOnly.deinit(allocator);
+    try std.testing.expectEqual(PlanKind.composite_eq, prefixOnly.kind);
+    try std.testing.expectEqualStrings("city_age", prefixOnly.indexName.?);
+    try std.testing.expectEqual(@as(usize, 1), prefixOnly.matchedFields);
+    try std.testing.expectEqual(@as(usize, 2), prefixOnly.totalFields);
+
+    // Both columns pinned by equality -> composite_eq, fully matched.
+    const eqPreds = [_]Predicate{
+        .{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } },
+        .{ .eq = .{ .field = "age", .value = .{ .int = 30 } } },
+    };
+    var fullEq = try table.explain(allocator, .{ .and_ = &eqPreds });
+    defer fullEq.deinit(allocator);
+    try std.testing.expectEqual(PlanKind.composite_eq, fullEq.kind);
+    try std.testing.expectEqual(@as(usize, 2), fullEq.matchedFields);
+
+    // Leading column pinned by equality, next column range-bound -> composite_range.
+    const rangePreds = [_]Predicate{
+        .{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } },
+        .{ .gte = .{ .field = "age", .value = .{ .int = 18 } } },
+    };
+    var rangeMatch = try table.explain(allocator, .{ .and_ = &rangePreds });
+    defer rangeMatch.deinit(allocator);
+    try std.testing.expectEqual(PlanKind.composite_range, rangeMatch.kind);
+    try std.testing.expectEqualStrings("city_age", rangeMatch.indexName.?);
+    try std.testing.expectEqual(@as(usize, 1), rangeMatch.matchedFields);
+}
+
+test "explain never mutates the table (const receiver, no rows touched)" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var table = try Table.open(&db, allocator, "users");
+    defer table.deinit();
+    try table.createIndex("city");
+    try table.put(.{ .int = 1 }, &.{.{ .name = "city", .value = .{ .text = "warsaw" } }});
+
+    const pred = Predicate{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } };
+    const constTable: *const Table = &table;
+    var plan = try constTable.explain(allocator, pred);
+    defer plan.deinit(allocator);
+    try std.testing.expectEqual(PlanKind.index_eq, plan.kind);
+
+    // The row explain() was asked about is still there afterwards.
+    const rows = try table.select(pred);
+    defer freeRows(allocator, rows);
+    try std.testing.expectEqual(@as(usize, 1), rows.len);
+}
+
+test "updateWhere applies a set clause to every matching row and reports the count" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var table = try Table.open(&db, allocator, "users");
+    defer table.deinit();
+    try table.put(.{ .int = 1 }, &.{ .{ .name = "city", .value = .{ .text = "warsaw" } }, .{ .name = "active", .value = .{ .int = 0 } } });
+    try table.put(.{ .int = 2 }, &.{ .{ .name = "city", .value = .{ .text = "warsaw" } }, .{ .name = "active", .value = .{ .int = 0 } } });
+    try table.put(.{ .int = 3 }, &.{ .{ .name = "city", .value = .{ .text = "krakow" } }, .{ .name = "active", .value = .{ .int = 0 } } });
+
+    const pred = Predicate{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } };
+    const updated = try table.updateWhere(pred, &.{.{ .name = "active", .value = .{ .int = 1 } }});
+    try std.testing.expectEqual(@as(usize, 2), updated);
+
+    var row1 = (try table.get(.{ .int = 1 })).?;
+    defer row1.deinit();
+    try std.testing.expectEqual(Value{ .int = 1 }, row1.get("active").?);
+    try std.testing.expectEqualStrings("warsaw", row1.get("city").?.text);
+
+    var row3 = (try table.get(.{ .int = 3 })).?;
+    defer row3.deinit();
+    try std.testing.expectEqual(Value{ .int = 0 }, row3.get("active").?);
+}
+
+test "updateWhere with a null predicate touches every row" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var table = try Table.open(&db, allocator, "counters");
+    defer table.deinit();
+    try table.put(.{ .int = 1 }, &.{.{ .name = "n", .value = .{ .int = 5 } }});
+    try table.put(.{ .int = 2 }, &.{.{ .name = "n", .value = .{ .int = 7 } }});
+
+    const updated = try table.updateWhere(null, &.{.{ .name = "flag", .value = .{ .text = "seen" } }});
+    try std.testing.expectEqual(@as(usize, 2), updated);
+
+    var row1 = (try table.get(.{ .int = 1 })).?;
+    defer row1.deinit();
+    try std.testing.expectEqualStrings("seen", row1.get("flag").?.text);
+    // Untouched fields survive the merge.
+    try std.testing.expectEqual(Value{ .int = 5 }, row1.get("n").?);
+}
+
+test "updateWhere adds a brand new field a matched row never had" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var table = try Table.open(&db, allocator, "users");
+    defer table.deinit();
+    try table.put(.{ .int = 1 }, &.{.{ .name = "city", .value = .{ .text = "warsaw" } }});
+
+    _ = try table.updateWhere(null, &.{.{ .name = "tier", .value = .{ .text = "gold" } }});
+
+    var row = (try table.get(.{ .int = 1 })).?;
+    defer row.deinit();
+    try std.testing.expectEqualStrings("gold", row.get("tier").?.text);
+    try std.testing.expectEqualStrings("warsaw", row.get("city").?.text);
+}
+
+test "updateWhere keeps a secondary index in sync when the indexed field changes" {
+    const allocator = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var db = try openTemp(&tmp, allocator);
+    defer db.close();
+
+    var table = try Table.open(&db, allocator, "users");
+    defer table.deinit();
+    try table.createIndex("city");
+    try table.put(.{ .int = 1 }, &.{.{ .name = "city", .value = .{ .text = "warsaw" } }});
+
+    const updated = try table.updateWhere(
+        .{ .eq = .{ .field = "city", .value = .{ .text = "warsaw" } } },
+        &.{.{ .name = "city", .value = .{ .text = "krakow" } }},
+    );
+    try std.testing.expectEqual(@as(usize, 1), updated);
+