sql - Why are batch inserts/updates faster? How do batch updates work?

Question

Why are batch inserts faster? Is it because the connection and setup overhead for inserting a single row is the same for a set of rows? What other factors make batch inserts faster?

How do batch updates work? Assuming the table has no uniqueness constraints, insert statements don't really have any effect on other insert statements in the batch. However, during batch updates, an update can alter the state of the table and hence can affect the outcome of other update queries in the batch.

I know that batch insert queries have a syntax where you have all the insert values in one big query. How do batch update queries look like? For e.g. if i have single update queries of the form:

update <table> set <column>=<expression> where <condition1>
update <table> set <column>=<expression> where <condition2>
update <table> set <column>=<expression> where <condition3>
update <table> set <column>=<expression> where <condition4>

What happens when they are used in a batch. What will the single query look like ?

And are batch inserts & updates part of the SQL standard?

Answer 1

I was looking for an answer on the same subject, about "bulk/batch" update. People often describe the problem by comparing it with insert clause with multiple value sets (the "bulk" part).

INSERT INTO mytable (mykey, mytext, myint)
VALUES 
  (1, 'text1', 11),
  (2, 'text2', 22),
  ...

Clear answer was still avoiding me, but I found the solution here: http://www.postgresql.org/docs/9.1/static/sql-values.html

To make it clear:

UPDATE mytable
SET 
  mytext = myvalues.mytext,
  myint = myvalues.myint
FROM (
  VALUES
    (1, 'textA', 99),
    (2, 'textB', 88),
    ...
) AS myvalues (mykey, mytext, myint)
WHERE mytable.mykey = myvalues.mykey

It has the same property of being "bulk" aka containing alot of data with one statement.