The segment_* tables contain memory allocation statistics for the Greenplum Database segment instances. This tracks the amount of memory consumed by all postgres processes of a particular segment instance, and the remaining amount of memory available to a segment as per the setting of the postgresql.conf configuration parameter: gp_vmem_protect_limit. Query processes that cause a segment to exceed this limit will be cancelled in order to prevent system-level out-of-memory errors. See the Greenplum Database Reference Guide for more information about this parameter.

There are three segment tables, all having the same columns:

• segment_now is an external table whose data files are stored in $MASTER_DATA_DIRECTORY/gpperfmon/data. Current memory allocation data is stored in segment_now during the period between data collection from the Command Center agents and automatic commitment to the segment_history table.
• segment_tail is an external table whose data files are stored in $MASTER_DATA_DIRECTORY/gpperfmon/data. This is a transitional table for memory allocation data that has been cleared from segment_now but has not yet been committed to segment_history. It typically only contains a few minutes worth of data.
• segment_history is a regular table that stores historical memory allocation metrics. It is pre-partitioned into monthly partitions. Partitions are automatically added in two month increments as needed. Administrators must drop old partitions for the months that are no longer needed.

A particular segment instance is identified by its hostname and dbid (the unique segment identifier as per the gp_segment_configuration system catalog table).

See also the views memory_info and dynamic_memory_info for aggregated memory allocation and utilization by host.