" 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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