Identifiers for segments are typically constructed using the segment datasource, interval start time (in ISO 8601 format), interval end time (in ISO 8601 format), and a version. If data is additionally sharded beyond a time range, the segment identifier will also contain a partition number.
The `smoosh` files represent multiple files "smooshed" together in order to minimize the number of file descriptors that must be open to house the data. They are files of up to 2GB in size (to match the limit of a memory mapped ByteBuffer in Java). The `smoosh` files house individual files for each of the columns in the data as well as an `index.drd` file with extra metadata about the segment.
There is also a special column called `__time` that refers to the time column of the segment. This will hopefully become less and less special as the code evolves, but for now it’s as special as my Mommy always told me I am.
A ColumnDescriptor is essentially an object that allows us to use jackson’s polymorphic deserialization to add new and interesting methods of serialization with minimal impact to the code. It consists of some metadata about the column (what type is it, is it multi-valued, etc.) and then a list of serde logic that can deserialize the rest of the binary.
Sharding Data to Create Segments
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### Sharding Data by Dimension
If the cumulative total number of rows for the different values of a given column exceed some configurable threshold, multiple segments representing the same time interval for the same datasource may be created. These segments will contain some partition number as part of their identifier. Sharding by dimension reduces some of the the costs associated with operations over high cardinality dimensions.
