Fix distinct by single LowCardinality numeric column.

dbms/src/Columns/ColumnLowCardinality.h

@@ -133,7 +133,7 @@ public:
     }
 
     bool valuesHaveFixedSize() const override { return getDictionary().valuesHaveFixedSize(); }
-    bool isFixedAndContiguous() const override { return getDictionary().isFixedAndContiguous(); }
+    bool isFixedAndContiguous() const override { return false; }
     size_t sizeOfValueIfFixed() const override { return getDictionary().sizeOfValueIfFixed(); }
     bool isNumeric() const override { return getDictionary().isNumeric(); }
     bool lowCardinality() const override { return true; }

dbms/src/Interpreters/SetVariants.cpp

@@ -137,7 +137,7 @@ typename SetVariantsTemplate<Variant>::Type SetVariantsTemplate<Variant>::choose
     }
 
     /// If there is one numeric key that fits into 64 bits
-    if (keys_size == 1 && nested_key_columns[0]->isNumeric())
+    if (keys_size == 1 && nested_key_columns[0]->isNumeric() && !nested_key_columns[0]->lowCardinality())
     {
         size_t size_of_field = nested_key_columns[0]->sizeOfValueIfFixed();
         if (size_of_field == 1)

dbms/tests/queries/0_stateless/00800_low_cardinality_distinct_numeric.reference

@@ -0,0 +1,123 @@
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+15
+16
+17
+18
+19
+20
+21
+22
+23
+24
+25
+26
+27
+28
+29
+30
+31
+32
+33
+34
+35
+36
+37
+38
+39
+40
+41
+42
+43
+44
+45
+46
+47
+48
+49
+50
+51
+52
+53
+54
+55
+56
+57
+58
+59
+60
+61
+62
+63
+64
+65
+66
+67
+68
+69
+70
+71
+72
+73
+74
+75
+76
+77
+78
+79
+80
+81
+82
+83
+84
+85
+86
+87
+88
+89
+90
+91
+92
+93
+94
+95
+96
+97
+98
+99
+100
+101
+102
+103
+104
+105
+106
+107
+108
+109
+110
+111
+112
+113
+114
+115
+116
+117
+118
+119
+120
+121
+122

dbms/tests/queries/0_stateless/00800_low_cardinality_distinct_numeric.sql

@@ -0,0 +1,7 @@
+set allow_experimental_low_cardinality_type = 1;
+drop table if exists test.lc;
+create table test.lc (val LowCardinality(UInt64)) engine = MergeTree order by val;
+insert into test.lc select number % 123 from system.numbers limit 100000;
+select distinct(val) from test.lc order by val;
+drop table if exists test.lc;
+