I wish to reduce the size of my test database by pruning inactive users. In Prune users, I selected the following parameters:
Last active: Before 2014-06-30
Delete pruned user posts: Yes
Deactivate or delete: Delete
After waiting 15 minutes, I received the confirmation page listing 33526 members, all boxes are checked.
Then, are you sure you wish to carry out this operation? Yes
I am immediately taken the the Forums --> Prune forum entry screen in the administration control panel. There are no messages, no prune operation is performed.
The prune operation has failed silently. No other problems are experienced.
PostgreSQL database size: 12 GB
Total posts: 2076723
Total topics: 136982
Total members 37742
I attempted a similar operation on my 3.0.12 database. The prune operation starts, then fails 2 hours later with:
SQL ERROR [ postgres ]
ERROR: stack depth limit exceeded HINT: Increase the configuration parameter "max_stack_depth" (currently 2048kB), after ensuring the platform's stack depth limit is adequate. 
SELECT search_key FROM search_results WHERE search_keywords...(a very long message)
FILE: (not given by php)
LINE: (not given by php)
Based on the above results, the phpBB 3.1.1 operation should have failed with an error message.
This problem is posted in https://www.phpbb.com/community/viewtopic.php?f=466&t=2271696.