fix: colDataKeepFirstNRows reset varmeta.length when all kept rows ar…

source/common/src/tdatablock.c

@@ -2496,7 +2496,10 @@ static void colDataKeepFirstNRows(SColumnInfoData* pColInfoData, size_t n, size_
         }
       }
       if (newLen <= -1) {
-        uFatal("colDataKeepFirstNRows: newLen:%d  old:%d", newLen, pColInfoData->varmeta.length);
+        // All kept rows are NULL — no valid var data belongs to them.
+        uDebug("colDataKeepFirstNRows: all kept rows are NULL, newLen:%d old:%d, reset length to 0",
+               newLen, pColInfoData->varmeta.length);
+        pColInfoData->varmeta.length = 0;
       } else {
         pColInfoData->varmeta.length = newLen;
       }

source/libs/decimal/src/detail/intx/int128.hpp

@@ -760,7 +760,9 @@ struct numeric_limits<intx::uint<N>>
     static constexpr bool is_bounded = true;
     static constexpr bool is_modulo = true;
     static constexpr int digits = CHAR_BIT * sizeof(type);
-    static constexpr int digits10 = int(0.3010299956639812 * digits);
+    // Maximum number of base-10 digits that can be represented by this type.
+    // Computed as floor(digits * log10(2)) where log10(2) ≈ 0.30102999566398119.
+    static constexpr int digits10 = static_cast<int>(0.30102999566398119521373889472449 * digits);
     static constexpr int max_digits10 = 0;
     static constexpr int radix = 2;
     static constexpr int min_exponent = 0;

test/cases/01-DataTypes/test_null_column.py

@@ -30,6 +30,80 @@ def test_null_column(self):
         self.NullColSma()
         tdStream.dropAllStreamsAndDbs()
 
+    def test_null_varchar_limit(self):
+        """NULL: varchar column with LIMIT triggers blockDataKeepFirstNRows all-NULL path
+
+        When the first N rows of a variable-length column are all NULL and a
+        LIMIT N query is executed, blockDataKeepFirstNRows must set
+        varmeta.length to 0 instead of keeping the stale value.
+
+        Since: v3.0.0.0
+
+        Labels: common,ci
+
+        Jira: TD-32818
+
+        History:
+            - 2026-4-3 Added to cover colDataKeepFirstNRows all-NULL var-data fix
+
+        """
+
+        tdSql.prepare("db_null_limit", drop=True)
+        tdSql.execute("use db_null_limit")
+
+        # Create a table with multiple var-length column types
+        tdSql.execute(
+            "create table t_var_null (ts timestamp, c1 varchar(64), c2 nchar(64), c3 int)"
+        )
+
+        # Insert rows: first 5 rows have NULL varchar/nchar, last 3 have values
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:01', NULL, NULL, 1)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:02', NULL, NULL, 2)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:03', NULL, NULL, 3)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:04', NULL, NULL, 4)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:05', NULL, NULL, 5)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:06', 'hello', 'world', 6)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:07', 'foo', 'bar', 7)")
+        tdSql.execute("insert into t_var_null values ('2024-01-01 00:00:08', 'test', 'data', 8)")
+
+        # LIMIT 3: keep only the first 3 rows (all NULL in c1/c2)
+        # This triggers blockDataKeepFirstNRows where all kept var-data rows are NULL
+        tdSql.query("select * from t_var_null order by ts limit 3")
+        tdSql.checkRows(3)
+        tdSql.checkData(0, 1, None)
+        tdSql.checkData(1, 1, None)
+        tdSql.checkData(2, 1, None)
+        tdSql.checkData(0, 2, None)
+        tdSql.checkData(1, 2, None)
+        tdSql.checkData(2, 2, None)
+        tdSql.checkData(0, 3, 1)
+        tdSql.checkData(1, 3, 2)
+        tdSql.checkData(2, 3, 3)
+
+        # LIMIT 5: all 5 NULL rows
+        tdSql.query("select * from t_var_null order by ts limit 5")
+        tdSql.checkRows(5)
+        for i in range(5):
+            tdSql.checkData(i, 1, None)
+            tdSql.checkData(i, 2, None)
+
+        # LIMIT 6: 5 NULL + 1 non-NULL — boundary case
+        tdSql.query("select * from t_var_null order by ts limit 6")
+        tdSql.checkRows(6)
+        for i in range(5):
+            tdSql.checkData(i, 1, None)
+        tdSql.checkData(5, 1, "hello")
+        tdSql.checkData(5, 2, "world")
+
+        # Also test with OFFSET to hit blockDataTrimFirstRows + blockDataKeepFirstNRows
+        tdSql.query("select * from t_var_null order by ts limit 2 offset 4")
+        tdSql.checkRows(2)
+        tdSql.checkData(0, 1, None)   # row index 4 — still NULL
+        tdSql.checkData(1, 1, "hello")  # row index 5
+
+        tdLog.info("test_null_varchar_limit passed")
+        tdStream.dropAllStreamsAndDbs()
+
     def ComputeNull(self):
         dbPrefix = "db"
         tbPrefix = "tb"