问题： ClickHouse 数据库引擎，默认的到底是 Atomic 还是 Ordinary？

I found this comment in source file: src/Databases/DatabaseAtomic.h

/// All tables in DatabaseAtomic have persistent UUID and store data in
/// /clickhouse_path/store/xxx/xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy/
/// where xxxyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy is UUID of the table.
/// RENAMEs are performed without changing UUID and moving table data.
/// Tables in Atomic databases can be accessed by UUID through DatabaseCatalog.
/// On DROP TABLE no data is removed, DatabaseAtomic just marks table as dropped
/// by moving metadata to /clickhouse_path/metadata_dropped/ and notifies DatabaseCatalog.
/// Running queries still may use dropped table. Table will be actually removed when it's not in use.
/// Allows to execute RENAME and DROP without IStorage-level RWLocks
class DatabaseAtomic : public DatabaseOrdinary
{
public:
    DatabaseAtomic(String name_, String metadata_path_, UUID uuid, const String & logger_name, ContextPtr context_);
    DatabaseAtomic(String name_, String metadata_path_, UUID uuid, ContextPtr context_);

    String getEngineName() const override { return "Atomic"; }
    UUID getUUID() const override { return db_uuid; }

ClickHouse 官网文档说：

默认情况下，ClickHouse使用Atomic数据库引擎。

源代码中说：

/** Default engine of databases.
  * It stores tables list in filesystem using list of .sql files,
  *  that contain declaration of table represented by SQL ATTACH TABLE query.
  */
class DatabaseOrdinary : public DatabaseOnDisk
{
public:
    DatabaseOrdinary(const String & name_, const String & metadata_path_, ContextPtr context);
    DatabaseOrdinary(
        const String & name_, const String & metadata_path_, const String & data_path_,
        const String & logger, ContextPtr context_);

    String getEngineName() const override { return "Ordinary"; }

Answer

Atomic Database Engine

In version 20.5 ClickHouse first introduced database engine=Atomic.

Since version 20.10 it is a default database engine (before engine=Ordinary was used).

Those 2 database engine differs in a way how they store data on a filesystem, and engine Atomic allows to resolve some of the issues existed in engine=Ordinary.

engine=Atomic supports

non-blocking drop table / rename table
tables delete (&detach) async (wait for selects finish but invisible for new selects)
atomic drop table (all files / folders removed)
atomic table swap (table swap by “EXCHANGE TABLES t1 AND t2;”)
rename dictionary / rename database
unique automatic UUID paths in FS and ZK for Replicated

https://kb.altinity.com/engines/altinity-kb-atomic-database-engine/