Slow postgresql query if where clause matches no rows - postgresql

Question

I have a simple table in a PostgreSQL 9.0.3 database that holds data polled from a wind turbine controller. Each row represents the value of a particular sensor at a particular time. Currently the table has around 90M rows:
wtdata=> \d integer_data
Table "public.integer_data"
Column | Type | Modifiers
--------+--------------------------+-----------
date | timestamp with time zone | not null
name | character varying(64) | not null
value | integer | not null
Indexes:
"integer_data_pkey" PRIMARY KEY, btree (date, name)
"integer_data_date_idx" btree (date)
"integer_data_name_idx" btree (name)
One query that I need is to find the last time that a variable was updated:
select max(date) from integer_data where name = '<name of variable>';
This query works fine when searching for a variable that exists in the table:
wtdata=> select max(date) from integer_data where name = 'STATUS_OF_OUTPUTS_UINT16';
max
------------------------
2011-04-11 02:01:40-05
(1 row)
However, if I try and search for a variable that doesn't exist in the table, the query hangs (or takes longer than I have patience for):
select max(date) from integer_data where name = 'Message';
I've let the query run for hours and sometimes days with no end in sight. There are no rows in the table with name = 'Message':
wtdata=> select count(*) from integer_data where name = 'Message';
count
-------
0
(1 row)
I don't understand why one query is fast and the other takes forever. Is the query somehow being forced to scan the entire table for some reason?
wtdata=> explain select max(date) from integer_data where name = 'Message';
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Result (cost=13.67..13.68 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.00..13.67 rows=1 width=8)
-> Index Scan Backward using integer_data_pkey on integer_data (cost=0.00..6362849.53 rows=465452 width=8)
Index Cond: ((date IS NOT NULL) AND ((name)::text = 'Message'::text))
(5 rows)
Here's the query plan for a fast query:
wtdata=> explain select max(date) from integer_data where name = 'STATUS_OF_OUTPUTS_UINT16';
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
Result (cost=4.64..4.65 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.00..4.64 rows=1 width=8)
-> Index Scan Backward using integer_data_pkey on integer_data (cost=0.00..16988170.38 rows=3659570 width=8)
Index Cond: ((date IS NOT NULL) AND ((name)::text = 'STATUS_OF_OUTPUTS_UINT16'::text))
(5 rows)

Answer 1

Change the primary key to (name,date).