doc: add examples of creative use of unique expression indexes

Unique expression indexes can constrain data in creative ways, so show
two examples.

Reported-by: Tuomas Leikola
Discussion: https://postgr.es/m/156760275564.1127.12321702656456074572@wrigleys.postgresql.org

Backpatch-through: 9.4

diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml
index 95c0a1926c5a6dd47581470f18d579608190254a..c54bf0dbbdb8c472aa266387c5939ef77ee44da7 100644 (file)
--- a/doc/src/sgml/indices.sgml
+++ b/doc/src/sgml/indices.sgml
@@ -705,6 +705,15 @@ CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
 
   
 
+  
+   Expression indexes also allow control over the scope of unique indexes.
+   For example, this unique index prevents duplicate integer values from
+   being stored in a double precision-typed column:
+
+CREATE UNIQUE INDEX test1_uniq_int ON tests ((floor(double_col)))
+
+  
+
   
    If we were to declare this index UNIQUE, it would prevent
    creation of rows whose col1 values differ only in case,
@@ -946,6 +955,16 @@ CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target)
     This is a particularly efficient approach when there are few
     successful tests and many unsuccessful ones.
    
+
+   
+    This index allows only one null in the indexed column by using a
+    partial index clause to process only null column values, and using
+    an expression index clause to index true instead
+    of null:
+
+CREATE UNIQUE INDEX tests_target_one_null ON tests ((target IS NULL)) WHERE target IS NULL;
+
+