By default all tables are created on all the segments, with this extension we could check or change the default behavior.

How does it work? Each table has a property numsegments to record how many segments its data is distributed one, the value is decided on CREATE TABLE according to an internal variable gp_create_table_default_numsegments, its value can be an integer number between 1 (the minimal segment count) and getgpsegmentCount() (all the primary segments in the cluster), or one of below magic numbsers (policies):

• GP_DEFAULT_NUMSEGMENTS_FULL: all the segments;
• GP_DEFAULT_NUMSEGMENTS_MINIMAL: the minimal set of segments;
• GP_DEFAULT_NUMSEGMENTS_RANDOM: pick a random set of segments each time;

This extension provides functions to get, set and reset this internal variable:

• gp_debug_set_create_table_default_numsegments(integer): set the default numsegments to an integer number between 1 and gp_num_contents_in_cluster;

• gp_debug_set_create_table_default_numsegments(text): set the default numsegments to one of the policies: FULL, MINIMAL and RANDOM;

• gp_debug_reset_create_table_default_numsegments(integer) and gp_debug_reset_create_table_default_numsegments(text): reset the default numsegments to the specified number or policy, the value can be reused later;

• gp_debug_reset_create_table_default_numsegments(): reset the default numsegments to the last value passed in above two forms, if there is no previous call to it the value is FULL;

• gp_debug_get_create_table_default_numsegments(): get the current number or policy of the default numsegments;

The changes are effective within the session.

As the name suggests, this extension should only be used for debugging purpose.