segment_*

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).

Column	Type	Description
`ctime`	timestamp(0) (without time zone)	The time the row was created.
`dbid`	int	The segment ID (`dbid` from `gp_segment_configuration`).
`hostname`	charvar(64)	The segment hostname.
`dynamic_memory_used`	bigint	The amount of dynamic memory (in bytes) allocated to query processes running on this segment.
`dynamic_memory_available`	bigint	The amount of additional dynamic memory (in bytes) that the segment can request before reaching the limit set by the `gp_vmem_protect_limit` parameter.

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