Fix function neighbor for LowCardinality argument.

src/Functions/neighbor.cpp

@@ -50,6 +50,8 @@ public:
 
     bool useDefaultImplementationForConstants() const override { return false; }
 
+    bool useDefaultImplementationForLowCardinalityColumns() const override { return false; }
+
     DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
     {
         size_t number_of_arguments = arguments.size();

tests/queries/0_stateless/01670_neighbor_lc_bug.sql

@@ -0,0 +1,43 @@
+SELECT
+    neighbor(n, -2) AS int,
+    neighbor(s, -2) AS str,
+    neighbor(lcs, -2) AS lowCstr
+FROM
+(
+    SELECT
+        number % 5 AS n,
+        toString(n) AS s,
+        CAST(s, 'LowCardinality(String)') AS lcs
+    FROM numbers(10)
+);
+
+drop table if exists neighbor_test;
+
+CREATE TABLE neighbor_test
+(
+    `rowNr` UInt8,
+    `val_string` String,
+    `val_low` LowCardinality(String)
+)
+ENGINE = MergeTree
+PARTITION BY tuple()
+ORDER BY rowNr;
+
+INSERT INTO neighbor_test VALUES (1, 'String 1', 'String 1'), (2, 'String 1', 'String 1'), (3, 'String 2', 'String 2');
+
+SELECT
+    rowNr,
+    val_string,
+    neighbor(val_string, -1) AS str_m1,
+    neighbor(val_string, 1) AS str_p1,
+    val_low,
+    neighbor(val_low, -1) AS low_m1,
+    neighbor(val_low, 1) AS low_p1
+FROM 
+(
+    SELECT *
+    FROM neighbor_test
+    ORDER BY val_string ASC
+) format PrettyCompact;
+
+drop table if exists neighbor_test;