Backport #19617 to 20.12: Fix function neighbor for LowCardinality argument.

src/Functions/neighbor.cpp

@@ -50,6 +50,10 @@ public:
 
     bool useDefaultImplementationForConstants() const override { return false; }
 
+    /// We do not use default implementation for LowCardinality because this is not a pure function.
+    /// If used, optimization for LC may execute function only for dictionary, which gives wrong result.
+    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.reference

+0		
+0		
+0	0	0
+1	1	1
+2	2	2
+3	3	3
+4	4	4
+0	0	0
+1	1	1
+2	2	2
+┌─rowNr─┬─val_string─┬─str_m1───┬─str_p1───┬─val_low──┬─low_m1───┬─low_p1───┐
+│     1 │ String 1   │          │ String 1 │ String 1 │          │ String 1 │
+│     2 │ String 1   │ String 1 │ String 2 │ String 1 │ String 1 │ String 2 │
+│     3 │ String 2   │ String 1 │          │ String 2 │ String 1 │          │
+└───────┴────────────┴──────────┴──────────┴──────────┴──────────┴──────────┘

tests/queries/0_stateless/01670_neighbor_lc_bug.sql

+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;