Doc: improve documentation about run-time pruning's effects on EXPLAIN.

Tatsuo Ishii complained that this para wasn't very intelligible.
Try to make it better.

Discussion: https://postgr.es/m/20191207.200500.989741087350666720[email protected]

diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml
index 715aff63c80adefb97c3643f80b7278715349621..0f61b0995d3d8abc678594e960b63d34647b3dc6 100644 (file)
--- a/doc/src/sgml/perform.sgml
+++ b/doc/src/sgml/perform.sgml
@@ -896,15 +896,16 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 WHERE unique1 < 100 AND unique2 > 9000
    
 
    
-    Generally, the EXPLAIN output will display details for
-    every plan node which was generated by the query planner.  However, there
-    are cases where the executor is able to determine that certain nodes are
-    not required; currently, the only node types to support this are the
-    Append and MergeAppend nodes.  These
-    node types have the ability to discard subnodes which they are able to
-    determine won't contain any records required by the query.  It is possible
-    to determine that nodes have been removed in this way by the presence of a
-    "Subplans Removed" property in the EXPLAIN output.
+    Normally, EXPLAIN will display every plan node
+    created by the planner.  However, there are cases where the executor
+    can determine that certain nodes need not be executed because they
+    cannot produce any rows, based on parameter values that were not
+    available at planning time.  (Currently this can only happen for child
+    nodes of an Append or MergeAppend node that is scanning a partitioned
+    table.)  When this happens, those plan nodes are omitted from
+    the EXPLAIN output and a Subplans
+    Removed: N annotation appears
+    instead.