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The HCatalog interface for Pig consists of HCatLoader and HCatStorer, which implement the Pig load and store interfaces respectively. HCatLoader accepts a table to read data from; you can indicate which partitions to scan by immediately following the load statement with a partition filter statement. HCatStorer accepts a table to write to and optionally a specification of partition keys to create a new partition. You can write to a single partition by specifying the partition key(s) and value(s) in the STORE clause; and you can write to multiple partitions if the partition key(s) are columns in the data being stored. HCatLoader is implemented on top of HCatInputFormat and HCatStorer is implemented on top of HCatOutputFormat. (See Load and Store Interfaces.)

The HCatalog interface for MapReduce — HCatInputFormat and HCatOutputFormat — is an implementation of Hadoop InputFormat and OutputFormat. HCatInputFormat accepts a table to read data from and optionally a selection predicate to indicate which partitions to scan. HCatOutputFormat accepts a table to write to and optionally a specification of partition keys to create a new partition. You can write to a single partition by specifying the partition key(s) and value(s) in the setOutput method; and you can write to multiple partitions if the partition key(s) are columns in the data being stored. (See Input and Output Interfaces.)

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Partitions contain records. Once a partition is created records cannot be added to it, removed from it, or updated in it. Partitions are multi-dimensional and not hierarchical. Records are divided into columns. Columns have a name and a datatype. HCatalog supports the same datatypes as Hive. See Load and Store Interfaces for more information about datatypes.

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