I had one mysql engine connection in clickhouse. 
I shutdown mysql aws RDS instance.

I updated clickhouse using apt get.
Then I restarted clickhouse. Clickhouse wont start.

`ClickHouse client version 20.7.2.30 (official build).`

```
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.13 (stretch)
Release:	9.13
Codename:	stretch
```

clickhouse server log

```
2020.09.03 12:31:01.937116 [ 947 ] {} <Debug> Application: Shut down storages.
2020.09.03 12:31:01.937566 [ 947 ] {} <Debug> Application: Destroyed global context.
2020.09.03 12:31:01.937693 [ 947 ] {} <Error> Application: DB::Exception: Cannot create MySQL database, because Poco::Exception. Code: 1000, e.code() = 2002, e.displayText() = mysqlxx::ConnectionFailed: Can't connect to MySQL server on '.....rds.amazonaws.com' (110) ((nullptr):0), Stack trace (when copying this message, always include the lines below):

0. Poco::Exception::Exception(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int) @ 0x1271b990 in /usr/bin/clickhouse
1. ? @ 0x12596556 in /usr/bin/clickhouse
2. mysqlxx::Pool::allocConnection(bool) @ 0x1259fb31 in /usr/bin/clickhouse
3. mysqlxx::Pool::initialize() @ 0x1259fedd in /usr/bin/clickhouse
4. mysqlxx::Pool::get() @ 0x125a00e8 in /usr/bin/clickhouse
5. DB::DatabaseMySQL::fetchTablesWithModificationTime() const @ 0xf46df4a in /usr/bin/clickhouse
6. DB::DatabaseMySQL::fetchTablesIntoLocalCache() const @ 0xf470b57 in /usr/bin/clickhouse
7. DB::DatabaseMySQL::empty() const @ 0xf470c15 in /usr/bin/clickhouse
8. DB::DatabaseFactory::getImpl(DB::ASTCreateQuery const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Context&) @ 0xf462839 in /usr/bin/clickhouse
9. DB::DatabaseFactory::get(DB::ASTCreateQuery const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Context&) @ 0xf463052 in /usr/bin/clickhouse
10. DB::InterpreterCreateQuery::createDatabase(DB::ASTCreateQuery&) @ 0xf455fc0 in /usr/bin/clickhouse
11. DB::InterpreterCreateQuery::execute() @ 0xf456f5e in /usr/bin/clickhouse
12. ? @ 0xf8338fa in /usr/bin/clickhouse
13. DB::loadMetadata(DB::Context&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) @ 0xf83441b in /usr/bin/clickhouse
14. DB::Server::main(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) @ 0xa495dd7 in /usr/bin/clickhouse
15. Poco::Util::Application::run() @ 0x1264a9e7 in /usr/bin/clickhouse
16. DB::Server::run() @ 0xa4558d9 in /usr/bin/clickhouse
17. mainEntryClickHouseServer(int, char**) @ 0xa44a993 in /usr/bin/clickhouse
18. main @ 0xa3dd919 in /usr/bin/clickhouse
19. __libc_start_main @ 0x202e1 in /lib/x86_64-linux-gnu/libc-2.24.so
20. _start @ 0xa3dd02e in /usr/bin/clickhouse
 (version 20.7.2.30 (official build))
2020.09.03 12:31:01.937740 [ 947 ] {} <Informa
```

context->getSettingsRef().postgresql_connection_pool_size,
            context->getSettingsRef().postgresql_connection_pool_wait_timeout);
        return std::make_shared<DatabasePostgreSQL>(
            context, metadata_path, engine_define, database_name, configuration, pool, use_table_cache);
    }
    else if (engine_name == "MaterializedPostgreSQL")
    {
        const ASTFunction * engine = engine_define->engine;
        if (!engine->arguments)
            throw Exception(ErrorCodes::BAD_ARGUMENTS, "Engine `{}` must have arguments", engine_name);
        ASTs & engine_args = engine->arguments->children;
        StoragePostgreSQLConfiguration configuration;
        if (auto named_collection = getExternalDataSourceConfiguration(engine_args, context, true))
        {
            auto [common_configuration, storage_specific_args] = named_collection.value();
            configuration.set(common_configuration);
            if (!storage_specific_args.empty())
                throw Exception(ErrorCodes::BAD_ARGUMENTS,
                                "MaterializedPostgreSQL Database requires only `host`, `port`, `database_name`, `username`, `password`.");
        }
        else
        {
            if (engine_args.size() != 4)
                throw Exception(ErrorCodes::BAD_ARGUMENTS,
                                "MaterializedPostgreSQL Database require `host:port`, `database_name`, `username`, `password`.");
            for (auto & engine_arg : engine_args)
                engine_arg = evaluateConstantExpressionOrIdentifierAsLiteral(engine_arg, context);
            auto parsed_host_port = parseAddress(safeGetLiteralValue<String>(engine_args[0], engine_name), 5432);
            configuration.host = parsed_host_port.first;
            configuration.port = parsed_host_port.second;