```
CREATE TABLE a
(
    `number` UInt64,
    `x` MATERIALIZED x
)
ENGINE = MergeTree
ORDER BY number;
-- that should fail!

insert into a values (1);
Received exception from server (version 21.7.1):
Code: 47. DB::Exception: Received from localhost:9000. DB::Exception: Missing columns: 'x' while processing query: 'CAST(x, 'UInt8') AS x', required columns: 'x' 'x'. 
```
{
    auto it = columns.get<1>().find(column_name);
    return it != columns.get<1>().end() && it->default_desc.expression;
}
std::optional<ColumnDefault> ColumnsDescription::getDefault(const String & column_name) const
{
    auto it = columns.get<1>().find(column_name);
    if (it != columns.get<1>().end() && it->default_desc.expression)
        return it->default_desc;
    return {};
}
bool ColumnsDescription::hasCompressionCodec(const String & column_name) const
{
    const auto it = columns.get<1>().find(column_name);
    return it != columns.get<1>().end() && it->codec != nullptr;
}
CompressionCodecPtr ColumnsDescription::getCodecOrDefault(const String & column_name, CompressionCodecPtr default_codec) const
{
    const auto it = columns.get<1>().find(column_name);
    if (it == columns.get<1>().end() || !it->codec)
        return default_codec;
    return CompressionCodecFactory::instance().get(it->codec, it->type, default_codec);
}
CompressionCodecPtr ColumnsDescription::getCodecOrDefault(const String & column_name) const