Add ability to change some types for primary key

src/Storages/MergeTree/MergeTreeData.cpp

@@ -5,6 +5,7 @@
 #include <DataTypes/DataTypeDateTime.h>
 #include <DataTypes/DataTypeEnum.h>
 #include <DataTypes/DataTypeNullable.h>
+#include <DataTypes/DataTypeLowCardinality.h>
 #include <DataTypes/NestedUtils.h>
 #include <Formats/FormatFactory.h>
 #include <Functions/FunctionFactory.h>
@@ -1316,10 +1317,10 @@ void MergeTreeData::dropIfEmpty()
 namespace
 {
 
-/// Conversion that is allowed for partition key.
+/// Conversion that is allowed for serializable key (primary key, sorting key).
-/// Partition key should be serialized in the same way after conversion.
+/// Key should be serialized in the same way after conversion.
 /// NOTE: The list is not complete.
-bool isSafeForPartitionKeyConversion(const IDataType * from, const IDataType * to)
+bool isSafeForKeyConversion(const IDataType * from, const IDataType * to)
 {
     if (from->getName() == to->getName())
         return true;
@@ -1346,6 +1347,12 @@ bool isSafeForPartitionKeyConversion(const IDataType * from, const IDataType * t
         return false;
     }
 
+    if (const auto * from_lc = typeid_cast<const DataTypeLowCardinality *>(from))
+        return from_lc->getDictionaryType()->equals(*to);
+
+    if (const auto * to_lc = typeid_cast<const DataTypeLowCardinality *>(to))
+        return to_lc->getDictionaryType()->equals(*from);
+
     return false;
 }
 
@@ -1540,7 +1547,7 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, const S
                     auto it = old_types.find(command.column_name);
 
                     assert(it != old_types.end());
-                    if (!isSafeForPartitionKeyConversion(it->second, command.data_type.get()))
+                    if (!isSafeForKeyConversion(it->second, command.data_type.get()))
                         throw Exception("ALTER of partition key column " + backQuoteIfNeed(command.column_name) + " from type "
                                 + it->second->getName() + " to type " + command.data_type->getName()
                                 + " is not safe because it can change the representation of partition key",
@@ -1554,9 +1561,11 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, const S
                 {
                     auto it = old_types.find(command.column_name);
                     assert(it != old_types.end());
-                    throw Exception("ALTER of key column " + backQuoteIfNeed(command.column_name) + " from type "
+                    if (!isSafeForKeyConversion(it->second, command.data_type.get()))
-                        + it->second->getName() + " to type " + command.data_type->getName() + " must be metadata-only",
+                        throw Exception("ALTER of key column " + backQuoteIfNeed(command.column_name) + " from type "
-                        ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
+                                    + it->second->getName() + " to type " + command.data_type->getName()
+                                    + " is not safe because it can change the representation of primary key",
+                            ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
                 }
             }
         }

tests/queries/0_stateless/01611_string_to_low_cardinality_key_alter.reference

@@ -0,0 +1,8 @@
+CREATE TABLE default.table_with_lc_key\n(\n    `enum_key` Enum8(\'y\' = 1, \'x\' = 2),\n    `lc_key` String,\n    `value` String\n)\nENGINE = MergeTree\nORDER BY (enum_key, lc_key)\nSETTINGS index_granularity = 8192
+y	hello	world
+CREATE TABLE default.table_with_lc_key\n(\n    `enum_key` Enum8(\'y\' = 1, \'x\' = 2, \'z\' = 3),\n    `lc_key` String,\n    `value` String\n)\nENGINE = MergeTree\nORDER BY (enum_key, lc_key)\nSETTINGS index_granularity = 8192
+y	hello	world
+CREATE TABLE default.table_with_lc_key\n(\n    `enum_key` Int8,\n    `lc_key` String,\n    `value` String\n)\nENGINE = MergeTree\nORDER BY (enum_key, lc_key)\nSETTINGS index_granularity = 8192
+1	hello	world
+CREATE TABLE default.table_with_string_key\n(\n    `int_key` Int8,\n    `str_key` LowCardinality(String),\n    `value` String\n)\nENGINE = MergeTree\nORDER BY (int_key, str_key)\nSETTINGS index_granularity = 8192
+1	hello	world

tests/queries/0_stateless/01611_string_to_low_cardinality_key_alter.sql

@@ -0,0 +1,67 @@
+DROP TABLE IF EXISTS table_with_lc_key;
+
+CREATE TABLE table_with_lc_key
+(
+    enum_key Enum8('x' = 2, 'y' = 1),
+    lc_key LowCardinality(String),
+    value String
+)
+ENGINE MergeTree()
+ORDER BY (enum_key, lc_key);
+
+INSERT INTO table_with_lc_key VALUES(1, 'hello', 'world');
+
+ALTER TABLE table_with_lc_key MODIFY COLUMN lc_key String;
+
+SHOW CREATE TABLE table_with_lc_key;
+
+DETACH TABLE table_with_lc_key;
+ATTACH TABLE table_with_lc_key;
+
+SELECT * FROM table_with_lc_key WHERE enum_key > 0 and lc_key like 'h%';
+
+ALTER TABLE table_with_lc_key MODIFY COLUMN enum_key Enum('x' = 2, 'y' = 1, 'z' = 3);
+ALTER TABLE table_with_lc_key MODIFY COLUMN enum_key Enum16('x' = 2, 'y' = 1, 'z' = 3); --{serverError 524}
+SHOW CREATE TABLE table_with_lc_key;
+
+DETACH TABLE table_with_lc_key;
+ATTACH TABLE table_with_lc_key;
+
+SELECT * FROM table_with_lc_key WHERE enum_key > 0 and lc_key like 'h%';
+
+ALTER TABLE table_with_lc_key MODIFY COLUMN enum_key Int8;
+
+SHOW CREATE TABLE table_with_lc_key;
+
+DETACH TABLE table_with_lc_key;
+ATTACH TABLE table_with_lc_key;
+
+SELECT * FROM table_with_lc_key WHERE enum_key > 0 and lc_key like 'h%';
+
+DROP TABLE IF EXISTS table_with_lc_key;
+
+
+DROP TABLE IF EXISTS table_with_string_key;
+CREATE TABLE table_with_string_key
+(
+    int_key Int8,
+    str_key String,
+    value String
+)
+ENGINE MergeTree()
+ORDER BY (int_key, str_key);
+
+INSERT INTO table_with_string_key VALUES(1, 'hello', 'world');
+
+ALTER TABLE table_with_string_key MODIFY COLUMN str_key LowCardinality(String);
+
+SHOW CREATE TABLE table_with_string_key;
+
+DETACH TABLE table_with_string_key;
+ATTACH TABLE table_with_string_key;
+
+SELECT * FROM table_with_string_key WHERE int_key > 0 and str_key like 'h%';
+
+ALTER TABLE table_with_string_key MODIFY COLUMN int_key Enum8('y' = 1, 'x' = 2); --{serverError 524}
+
+DROP TABLE IF EXISTS table_with_string_key;