"
Like many modern analytical engines [18, 20], Procella
does not use the conventional BTree style secondary indexes,
opting instead for light weight secondary structures such as
zone maps, bitmaps, bloom filters, partition and sort keys [1].
The metadata server serves this information during query
planning time. These secondary structures are collected
partly from the file headers during file registration, by the
registration server, and partly lazily at query evaluation time
by the data server. Schemas, table to file mapping, stats,
zone maps and other metadata are mostly stored in the
metadata store (in Bigtable [11] and Spanner [12])."
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