diff --git a/src/backend/partitioning/partprune.c b/src/backend/partitioning/partprune.c
index ff68dca..9998583 100644
--- a/src/backend/partitioning/partprune.c
+++ b/src/backend/partitioning/partprune.c
@@ -1,10 +1,36 @@
 /*-------------------------------------------------------------------------
  *
  * partprune.c
- *		Parses clauses attempting to match them up to partition keys of a
- *		given relation and generates a set of "pruning steps", which can be
- *		later "executed" either from the planner or the executor to determine
- *		the minimum set of partitions which match the given clauses.
+ *		This module implements partition pruning using the information
+ *		contained in table's partition descriptor.
+ *
+ * Functionality of this module can be invoked either from the planner or the
+ * executor.  During planning, clauses that can be matched to the table's
+ * partition key are turned into a set of "pruning steps", which are then
+ * executed to produce a set of indexes of partitions whose bounds satisfy the
+ * set of clauses from which a given step was constructed.  When invoked from
+ * the executor, "pruning steps" that were generated during planning are
+ * executed using expressions whose values could only be known during
+ * execution, such as Params.
+ *
+ * There are two kinds of pruning steps -- a "base" pruning step, which
+ * contains information extracted from one of more OpExpr clauses that are
+ * matched to the (possibly multi-column) partition key, such as the
+ * expressions whose values to match against partition bounds and operator
+ * strategy to assume when determining the set of partition bounds that would
+ * match those expressions.  Whereas, a "combine" pruning step is constructed
+ * for BoolExpr clauses which simply combines the outputs of the steps
+ * corresponding to its argument clauses using the appropriate combination
+ * method.  All steps that are constructed are executed in succession such
+ * that for any "combine" step, all of the steps whose output it depends on
+ * are executed first and their ouput preserved.  Refer to
+ * gen_partprune_steps_internal() for more details.
+ *
+ * A base pruning step may consist of Param expressions whose values are
+ * only made available during execution, in which case, pruning cannot occur
+ * during planning itself or it occurs only partially with other expressions
+ * whose values are known during planning.  Such steps are included with the
+ * plan, so the they can be executed at an appropriate time during execution.
  *
  * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California