Explain automatic creation (or lack of it) of indexes for the various types

of constraints.

Kevin Grittner

diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 8f42a8d6304bf906dde81594416a019ef57d66a4..261f675368e633e0597f14ce335d0fc7dea7bdd7 100644 (file)
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -1,4 +1,4 @@
-
+
 
 
  Data Definition
@@ -544,6 +544,11 @@ CREATE TABLE products (
 
    
 
+   
+    Adding a unique constraint will automatically create a unique btree
+    index on the column or group of columns used in the constraint.
+   
+
    
     null value
     with unique constraints
@@ -622,6 +627,11 @@ CREATE TABLE example (
     uniquely.
    
 
+   
+    Adding a primary key will automatically create a unique btree index
+    on the column or group of columns used in the primary key.
+   
+
    
     A table can have at most one primary key.  (There can be any number
     of unique and not-null constraints, which are functionally the same
@@ -831,6 +841,16 @@ CREATE TABLE order_items (
     column is changed (updated).  The possible actions are the same.
    
 
+   
+    Since a DELETE of a row from the referenced table
+    or an UPDATE of a referenced column will require
+    a scan of the referencing table for rows matching the old value, it
+    is often a good idea to index the referencing columns.  Because this
+    is not always needed, and there are many choices available on how
+    to index, declaration of a foreign key constraint does not
+    automatically create an index on the referencing columns.
+   
+
    
     More information about updating and deleting data is in 
     linkend="dml">.
@@ -875,6 +895,11 @@ CREATE TABLE circles (
     See also CREATE
     TABLE ... CONSTRAINT ... EXCLUDE for details.
    
+
+   
+    Adding an exclusion constraint will automatically create an index
+    of the type specified in the constraint declaration.
+