CPU hotspot on `ColumnString::index`

[cangyin]

This is a flamegraph for INSERT INTO SummingMergeTree SELECT ... query. It shows ClickHouse is spending significant CPU time on DB::ColumnString::index

Both DB::ColumnString::index() in the graph are called by convertToFullColumnIfLowCardinality() in IMergeTreeIndexAggregator::update()

[cangyin]

Because convertToFullColumnIfLowCardinality is called (converting the whole column with heavy memory copy operations) for each data granule when update-ing BloomFilter/Set indexes.