```
DROP TABLE IF EXISTS foo;

CREATE TABLE foo (timestamp DateTime, x UInt64)
ENGINE = MergeTree PARTITION BY toYYYYMMDD(timestamp)
ORDER BY (timestamp);

INSERT INTO foo (timestamp, x) SELECT toDateTime('2020-01-01 00:05:00'), number from system.numbers_mt LIMIT 100;

SELECT count() FROM system.mutations; -- ok

ALTER TABLE foo UPDATE x = 1 WHERE x = (SELECT x from foo WHERE x = 1);

SELECT count() FROM system.mutations; -- stuck

```
else
            ++it;
    }
    
    
    if (storage)
    {
        const auto storage_virtuals = storage->getVirtuals();
        for (auto it = unknown_required_source_columns.begin(); it != unknown_required_source_columns.end();)
        {
            auto column = storage_virtuals.tryGetByName(*it);
            if (column)
            {
                source_columns.push_back(*column);
                unknown_required_source_columns.erase(it++);
            }
            else
                ++it;
        }
    }
    if (!unknown_required_source_columns.empty())
    {
        WriteBufferFromOwnString ss;
        ss << "Missing columns:";
        for (const auto & name : unknown_required_source_columns)
            ss << " '" << name << "'";
        ss << " while processing query: '" << queryToString(query) << "'";
        ss << ", required columns:";
        for (const auto & name : columns_context.requiredColumns())
            ss << " '" << name << "'";