More about chained mode and isolation levels.

diff --git a/doc/src/sgml/ref/begin.sgml b/doc/src/sgml/ref/begin.sgml
index cd9842a87dea89aa663693e8900372b3a9c6af8b..62e5b88d0a75c7ea4b0a20650eb434c33999aba5 100644 (file)
--- a/doc/src/sgml/ref/begin.sgml
+++ b/doc/src/sgml/ref/begin.sgml
@@ -10,7 +10,7 @@
    BEGIN WORK
   
   
-   Begins a transaction
+   Begins a transaction in chained mode
   
   
   
@@ -78,15 +78,31 @@ BEGIN [ WORK | TRANSACTION ]
    Description
   
   
-   BEGIN initiates a user transaction
- which Postgres will
-   guarantee is serializable with respect to all concurrently
-   executing transactions. Postgres uses two-phase
- locking
-   to perform this task. If the transaction is committed,
-   Postgres will ensure either that all updates are
- done or else
-   that none of
+   By default, Postgres executes transactions
+   in unchained mode (also known as autocommit feature in other DBMSes).
+   In other words, each user statement is executed in its own transaction
+   and commit is implicit (if execution was successfull).
+   BEGIN initiates a user transaction in chained mode,
+   i.e. all user statements after BEGIN command will
+   be executed in single transaction untill explicit COMMIT, ROLLBACK
+   or execution abort. Statements in chained mode are executed much faster, 
+   because of transaction start/commit requires significant CPU and disk 
+   activity. This mode is also required for consistency when changing 
+   one of related tables.
+  
+  
+   Default transaction isolation level in Postgres
+   is READ COMMITTED one, when queries inside transaction see only changes
+   committed before query execution. So, you have to use 
+   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
+   command just after BEGIN if you need in better transaction isolation. 
+   In SERIALIZABLE mode queries will see only changes committed before entire
+   transaction began (actually, before execution of first DML statement
+   in serializable transaction).
+  
+  
+   If the transaction is committed, Postgres 
+   will ensure either that all updates are done or else that none of
    them are done. Transactions have the standard ACID
    (atomic, consistent, isolatable, and durable) property.
   
@@ -105,12 +121,12 @@ BEGIN [ WORK | TRANSACTION ]
 
    
     Refer to the LOCK statement for further information
- about locking tables inside a transaction.
+    about locking tables inside a transaction.
    
    
    
     Use COMMIT or ROLLBACK
- to terminate a transaction.
+    to terminate a transaction.
    
   
  
@@ -133,7 +149,7 @@ BEGIN WORK;
   
   
     BEGIN
- is a Postgres language extension.
+    is a Postgres language extension.
   
   
      
@@ -144,9 +160,12 @@ BEGIN WORK;
    
     
     There is no explicit BEGIN WORK command in SQL92;
- transaction initiation
-    is always implicit and it terminates either with a COMMIT or with
-    a ROLLBACK statement.
+    transaction initiation is always implicit and it terminates either 
+    with a COMMIT or with a ROLLBACK statement.
+   
+   
+    SQL92 also requires SERIALIZABLE to be default
+    transaction isolation level.