TiFlash fork from Clickhouse around late 2018. There are lots of useless codes for TiFlash.

• Remove StorageKafka, StorageMySQL, StorageODBC and CapnProtoRowInputStream Remove useless codes #1279
• (Mainly) Remove documents for Clickhouse Remove useless files for TiFlash #1596
• Remove ReplicatedMergeTree family (TiFlash does not rely on those storage engines to do replication) Remove ReplicatedMergeTree family #1805
• Remove MergeTree family #3481
• Remove StorageDistributed #3484
• Automatically update related generated-cpp files after kvproto/tipb get updated Better integration with tipb, kvproto #2110
• Remove some useless dependency for TiFlash.
  • kafka Reduction of extraneous compoents  #2022
  • MongoDB, MySQL, odbc Reduction of extraneous compoents  #2022
  • XML
    Clickhouse uses XML as the configuration file format but TiFlash uses toml. Seems that TiFlash do not rely on XML anymore.
  • Zookeeper Remove ClickHouse DDL worker #2253 Remove ZooKeeper #2255
    Clickhouse use zookeeper for replication, but TiFlash rely on PD and TiKV's Raft layer
• Remove Analyzers Remove useless Analyzers #2497 Remove uesless Analyzers #2549
• Remove ClickHouse integration test cases under dbms/tests #3485
• Remove Interpreters/{Users,SecurityManager,Quota} and related logics #3486
  These are outdated permission management, resource management from ClickHouse
• Remove unused configuration "interserver_http_port" #3491

Something not sure:

• Remove redundant data types so that we don't need to instantiate some template class for these data types, which may greatly reduce our compile time
  • "DataTypeDate"/"DateTypeDateTime"
    We use "DataTypeMyDate"/"DataTypeMyDatetime" for MySQL/TiDB-compatibility, these original data types ported from Clickhouse are useless for TiFlash
  • "DataTypeTuple"/"DataTypeArray"/"DataTypeSet"
    We don't use these data type

Possible it is good for speeding up our CI building step #1390.