Update README

Author: ildus

README.md

@@ -68,3 +68,62 @@ If we keep the plan for the query and enable it to be used also for the followin
 select query_hash from sr_plans where query_hash=1000+_p(11);
 select query_hash from sr_plans where query_hash=1000+_p(-5);
 ```
+
+## Explain
+
+It is possible to see saved plans by using `show_plan` function. It requires
+knowing query hash which could be fetched from `sr_plans` table.
+
+Examples:
+
+```SQL
+SELECT show_plan(1);
+                  show_plan                   
+----------------------------------------------
+ ("Seq Scan on public.explain_test")
+ ("  Output: test_attr1, test_attr2")
+ ("  Filter: (explain_test.test_attr1 = 10)")
+(3 rows)
+```
+
+Get second plan:
+
+```SQL
+SELECT show_plan(1, index := 2);
+                  show_plan                   
+----------------------------------------------
+ ("Seq Scan on public.explain_test")
+ ("  Output: test_attr1, test_attr2")
+ ("  Filter: (explain_test.test_attr1 = 10)")
+(3 rows)
+```
+
+Use another output format (supported formats are `json`, `text`, `xml`):
+
+```SQL
+SELECT show_plan(1, format := 'json');
+                      show_plan                       
+------------------------------------------------------
+ ("[                                                 +
+   {                                                 +
+     ""Plan"": {                                     +
+       ""Node Type"": ""Seq Scan"",                  +
+       ""Parallel Aware"": false,                    +
+       ""Relation Name"": ""explain_test"",          +
+       ""Schema"": ""public"",                       +
+       ""Alias"": ""explain_test"",                  +
+       ""Output"": [""test_attr1"", ""test_attr2""], +
+       ""Filter"": ""(explain_test.test_attr1 = 10)""+
+     }                                               +
+   }                                                 +
+ ]")
+(1 row)
+```
+
+## `pg_stat_statements` integration
+
+`sr_plans` table contains `query_id` columns which could be used to make
+joins with `pg_stat_statements` tables and views.
+
+Note: in `shared_preload_libraries` list `pg_stat_statements` should be
+specified after `sr_plan`.