create secondary index needs attention

[mdcallag]

My requests:

• Improve the docs, especially for create secondary index
• Explain the options (session variables) that can affect it including rocksdb_bulk_load_allow_sk, rocksdb_bulk_load, rocksdb_merge_combine_read_size and rocksdb_merge_buf_size.

My focus is on creating a secondary index -- alter table add index ...

Questions I have include:

• do any of the rocksdb_bulk_load* options have an impact
• how many allocations of size rocksdb_merge_combine_read_size are concurrently active? Is it one per alter table? Is it one per index per alter table? I want to avoid OOM. I have the same question about rocksdb_merge_buf_size.

From experiments, the rocksdb_bulk_load* options have no impact and the bulk load approach is always used.