diff --git a/src/ops/group.c b/src/ops/group.c
index d2402231..aa9509c7 100644
--- a/src/ops/group.c
+++ b/src/ops/group.c
@@ -34,6 +34,11 @@
 typedef struct {
     double sum_f, min_f, max_f, prod_f, first_f, last_f, sum_sq_f;
     int64_t sum_i, min_i, max_i, prod_i, first_i, last_i, sum_sq_i;
+    /* Parallel f64 sum of the integer stream — used by AVG so the
+     * mean of an i64 column whose sum exceeds 2^63 (e.g. ClickBench
+     * UserID, signed values around ±9e18 × 10^7 rows) stays accurate
+     * instead of being whatever (uint64) wrap left in sum_i. */
+    double sum_d;
     int64_t cnt;
     int64_t null_count;
     bool has_first;
@@ -44,6 +49,7 @@ static void reduce_acc_init(reduce_acc_t* acc) {
     acc->prod_f = 1.0; acc->first_f = 0; acc->last_f = 0; acc->sum_sq_f = 0;
     acc->sum_i = 0; acc->min_i = INT64_MAX; acc->max_i = INT64_MIN;
     acc->prod_i = 1; acc->first_i = 0; acc->last_i = 0; acc->sum_sq_i = 0;
+    acc->sum_d = 0;
     acc->cnt = 0; acc->null_count = 0; acc->has_first = false;
 }
 
@@ -92,6 +98,7 @@ static inline bool sym_lex_gt(int64_t a, int64_t b) { return sym_lex_lt(b, a); }
             (acc)->sum_i    = (int64_t)((uint64_t)(acc)->sum_i    + (uint64_t)v); \
             (acc)->sum_sq_i = (int64_t)((uint64_t)(acc)->sum_sq_i + (uint64_t)v * (uint64_t)v); \
             (acc)->prod_i   = (int64_t)((uint64_t)(acc)->prod_i   * (uint64_t)v); \
+            (acc)->sum_d   += (double)v; \
             if (v < (acc)->min_i) (acc)->min_i = v; \
             if (v > (acc)->max_i) (acc)->max_i = v; \
             if (!(acc)->has_first) { (acc)->first_i = v; (acc)->has_first = true; } \
@@ -159,6 +166,7 @@ static void reduce_range(ray_t* input, int64_t start, int64_t end,
             acc->sum_i    = (int64_t)((uint64_t)acc->sum_i    + (uint64_t)v);
             acc->sum_sq_i = (int64_t)((uint64_t)acc->sum_sq_i + (uint64_t)v * (uint64_t)v);
             acc->prod_i   = (int64_t)((uint64_t)acc->prod_i   * (uint64_t)v);
+            acc->sum_d   += (double)v;
             if (v < acc->min_i) acc->min_i = v;
             if (v > acc->max_i) acc->max_i = v;
             if (!acc->has_first) { acc->first_i = v; acc->has_first = true; }
@@ -262,6 +270,7 @@ static void reduce_merge(reduce_acc_t* dst, const reduce_acc_t* src, int8_t in_t
         dst->sum_i    = (int64_t)((uint64_t)dst->sum_i    + (uint64_t)src->sum_i);
         dst->sum_sq_i = (int64_t)((uint64_t)dst->sum_sq_i + (uint64_t)src->sum_sq_i);
         dst->prod_i   = (int64_t)((uint64_t)dst->prod_i   * (uint64_t)src->prod_i);
+        dst->sum_d   += src->sum_d;
         if (in_type == RAY_SYM) {
             /* Lex compare for SYM min/max (see sym_lex_lt). */
             if (src->cnt > 0) {
@@ -2051,7 +2060,7 @@ ray_t* exec_reduction(ray_graph_t* g, ray_op_t* op, ray_t* input) {
             /* COUNT returns total length including nulls — matches ray_count_fn's
              * "count all elements" semantics, not SQL's COUNT(col) non-null count. */
             case OP_COUNT: result = ray_i64(scan_n); break;
-            case OP_AVG:   result = merged.cnt > 0 ? ray_f64(in_type == RAY_F64 ? merged.sum_f / merged.cnt : (double)merged.sum_i / merged.cnt) : ray_typed_null(-RAY_F64); break;
+            case OP_AVG:   result = merged.cnt > 0 ? ray_f64(in_type == RAY_F64 ? merged.sum_f / merged.cnt : merged.sum_d / merged.cnt) : ray_typed_null(-RAY_F64); break;
             case OP_FIRST: result = merged.has_first ? (in_type == RAY_F64 ? ray_f64(merged.first_f) : reduction_i64_result(merged.first_i, in_type)) : ray_typed_null(-in_type); break;
             case OP_LAST:  result = merged.has_first ? (in_type == RAY_F64 ? ray_f64(merged.last_f) : reduction_i64_result(merged.last_i, in_type)) : ray_typed_null(-in_type); break;
             case OP_VAR: case OP_VAR_POP:
@@ -2060,7 +2069,7 @@ ray_t* exec_reduction(ray_graph_t* g, ray_op_t* op, ray_t* input) {
                 if (insufficient) { result = ray_typed_null(-RAY_F64); break; }
                 double mean, var_pop;
                 if (in_type == RAY_F64) { mean = merged.sum_f / merged.cnt; var_pop = merged.sum_sq_f / merged.cnt - mean * mean; }
-                else { mean = (double)merged.sum_i / merged.cnt; var_pop = (double)merged.sum_sq_i / merged.cnt - mean * mean; }
+                else { mean = merged.sum_d / merged.cnt; var_pop = (double)merged.sum_sq_i / merged.cnt - mean * mean; }
                 if (var_pop < 0) var_pop = 0;
                 double val;
                 if (op->opcode == OP_VAR_POP) val = var_pop;
@@ -2090,7 +2099,7 @@ ray_t* exec_reduction(ray_graph_t* g, ray_op_t* op, ray_t* input) {
         /* COUNT returns total length including nulls — matches ray_count_fn's
          * "count all elements" semantics, not SQL's COUNT(col) non-null count. */
         case OP_COUNT: return ray_i64(scan_n);
-        case OP_AVG:   return acc.cnt > 0 ? ray_f64(in_type == RAY_F64 ? acc.sum_f / acc.cnt : (double)acc.sum_i / acc.cnt) : ray_typed_null(-RAY_F64);
+        case OP_AVG:   return acc.cnt > 0 ? ray_f64(in_type == RAY_F64 ? acc.sum_f / acc.cnt : acc.sum_d / acc.cnt) : ray_typed_null(-RAY_F64);
         case OP_FIRST: return acc.has_first ? (in_type == RAY_F64 ? ray_f64(acc.first_f) : reduction_i64_result(acc.first_i, in_type)) : ray_typed_null(-in_type);
         case OP_LAST:  return acc.has_first ? (in_type == RAY_F64 ? ray_f64(acc.last_f) : reduction_i64_result(acc.last_i, in_type)) : ray_typed_null(-in_type);
         case OP_VAR: case OP_VAR_POP:
@@ -2099,7 +2108,7 @@ ray_t* exec_reduction(ray_graph_t* g, ray_op_t* op, ray_t* input) {
             if (insufficient) return ray_typed_null(-RAY_F64);
             double mean, var_pop;
             if (in_type == RAY_F64) { mean = acc.sum_f / acc.cnt; var_pop = acc.sum_sq_f / acc.cnt - mean * mean; }
-            else { mean = (double)acc.sum_i / acc.cnt; var_pop = (double)acc.sum_sq_i / acc.cnt - mean * mean; }
+            else { mean = acc.sum_d / acc.cnt; var_pop = (double)acc.sum_sq_i / acc.cnt - mean * mean; }
             if (var_pop < 0) var_pop = 0;
             double val;
             if (op->opcode == OP_VAR_POP) val = var_pop;