It is possible to trigger
MaterializedMySQLSyncThread::stopSynchronization() from the same thread
in case of some exception at startup, when some interpreter holds the
storage refcnt, and later MaterializedMySQLSyncThread will try to join
itself from the main thread.
Here is a stack trace for example:
<details>
4 0x000000000f7ae45c in Poco::Event::wait (this=0x7f1b90c38170) at ../contrib/poco/Foundation/include/Poco/Event.h:97
5 ThreadFromGlobalPool::join (this=0x7f1b90c23120) at ../src/Common/ThreadPool.h:210
6 DB::MaterializeMySQLSyncThread::stopSynchronization (this=0x7f1b8024ca68) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:229
7 DB::MaterializeMySQLSyncThread::~MaterializeMySQLSyncThread (this=0x7f1b8024ca68) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:85
8 0x000000000f816dc8 in DB::DatabaseMaterializeMySQL<DB::DatabaseAtomic>::~DatabaseMaterializeMySQL (this=0x7f1b8024c918) at ../src/Databases/MySQL/DatabaseMaterializeMySQL.h:21
11 std::__1::shared_ptr<DB::IDatabase>::~shared_ptr (this=<optimized out>) at ../contrib/libcxx/include/memory:3212
12 0x000000000f8726a6 in DB::InterpreterCreateQuery::createTable (this=<optimized out>, create=...) at ../src/Interpreters/InterpreterCreateQuery.cpp:952
13 0x000000000f87735c in DB::InterpreterCreateQuery::execute (this=0x7f1aeef59860) at ../src/Interpreters/InterpreterCreateQuery.cpp:1225
14 0x000000000fe22253 in DB::executeQueryImpl (begin=<optimized out>, end=<optimized out>, context=..., internal=true, stage=DB::QueryProcessingStage::Complete, has_query_tail=<optimized out>, istr=<optimized out>) at ../src/Interpreters/executeQuery.cpp:574
15 0x000000000fe208e3 in DB::executeQuery (query=..., context=..., internal=<optimized out>, stage=DB::QueryProcessingStage::FetchColumns, may_have_embedded_data=<optimized out>) at ../src/Interpreters/executeQuery.cpp:933
16 0x000000000faafcde in DB::MySQLInterpreter::InterpreterMySQLDDLQuery<DB::MySQLInterpreter::InterpreterCreateImpl>::execute (this=<optimized out>) at ../src/Interpreters/MySQL/InterpretersMySQLDDLQuery.h:75
17 0x000000000faade78 in DB::InterpreterExternalDDLQuery::execute (this=<optimized out>) at ../src/Interpreters/InterpreterExternalDDLQuery.cpp:64
18 0x000000000fe22253 in DB::executeQueryImpl (begin=<optimized out>, end=<optimized out>, context=..., internal=true, stage=DB::QueryProcessingStage::Complete, has_query_tail=<optimized out>, istr=<optimized out>) at ../src/Interpreters/executeQuery.cpp:574
19 0x000000000fe208e3 in DB::executeQuery (query=..., context=..., internal=<optimized out>, stage=DB::QueryProcessingStage::FetchColumns, may_have_embedded_data=<optimized out>) at ../src/Interpreters/executeQuery.cpp:933
20 0x000000000f7ba406 in DB::tryToExecuteQuery (query_to_execute=..., query_context=..., database=..., comment=...) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:69
21 0x000000000f7d4b88 in DB::dumpDataForTables() (connection=..., query_prefix=..., database_name=..., mysql_database_name=..., context=..., is_cancelled=..., need_dumping_tables=...) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:326
22 DB::MaterializeMySQLSyncThread::prepareSynchronized(DB::MaterializeMetadata&)::$_1::operator()() const (this=<optimized out>) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:391
29 DB::commitMetadata() (function=..., persistent_tmp_path=..., persistent_path=...) at ../src/Databases/MySQL/MaterializeMetadata.cpp:197
30 0x000000000f80a000 in DB::MaterializeMetadata::transaction(DB::MySQLReplication::Position const&, std::__1::function<void ()> const&) (this=0x7f1b6375c3d8, position=..., fun=...) at ../src/Databases/MySQL/MaterializeMetadata.cpp:230
31 0x000000000f7b2231 in DB::MaterializeMySQLSyncThread::prepareSynchronized (this=0x7f1b8024ca68, metadata=...) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:388
32 0x000000000f7b169c in DB::MaterializeMySQLSyncThread::synchronization (this=0x7f1b8024ca68) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:180
33 0x000000000f7d4074 in DB::MaterializeMySQLSyncThread::startSynchronization()::$_0::operator()() const (this=<optimized out>) at ../src/Databases/MySQL/MaterializeMySQLSyncThread.cpp:236
</details>
Nested identical combinators (i.e. uniqCombinedIfIf) is not
supported (since they even don't work -- silently).
But non-identical does supported and works, for example
uniqCombinedIfMergeIf, it is useful in case when the underlying
storage stores AggregateFunction(uniqCombinedIf) and in SELECT you
need to filter aggregation result based on another column for
example.
Second If combinator can be useful to add condition based on another
column, when Merge combinator is applied for example (i.e.
uniqCombinedIfMergeIf(state_of_merge_that_is_stored_in_table, event_date = today())).
