I\'m dealing with a Postgres table (called \"lives\") that contains records with columns for time_stamp, usr_id, transaction_id, and lives_remaining. I need a query that wil
Another solution you might find useful.
SELECT t.*
FROM
(SELECT
*,
ROW_NUMBER() OVER(PARTITION BY usr_id ORDER BY time_stamp DESC) as r
FROM lives) as t
WHERE t.r = 1
I think you've got one major problem here: there's no monotonically increasing "counter" to guarantee that a given row has happened later in time than another. Take this example:
timestamp lives_remaining user_id trans_id
10:00 4 3 5
10:00 5 3 6
10:00 3 3 1
10:00 2 3 2
You cannot determine from this data which is the most recent entry. Is it the second one or the last one? There is no sort or max() function you can apply to any of this data to give you the correct answer.
Increasing the resolution of the timestamp would be a huge help. Since the database engine serializes requests, with sufficient resolution you can guarantee that no two timestamps will be the same.
Alternatively, use a trans_id that won't roll over for a very, very long time. Having a trans_id that rolls over means you can't tell (for the same timestamp) whether trans_id 6 is more recent than trans_id 1 unless you do some complicated math.
On a table with 158k pseudo-random rows (usr_id uniformly distributed between 0 and 10k, trans_id
uniformly distributed between 0 and 30),
By query cost, below, I am referring to Postgres' cost based optimizer's cost estimate (with Postgres' default xxx_cost
values), which is a weighed function estimate of required I/O and CPU resources; you can obtain this by firing up PgAdminIII and running "Query/Explain (F7)" on the query with "Query/Explain options" set to "Analyze"
usr_id
, trans_id
, time_stamp
))usr_id
, trans_id
))usr_id
, trans_id
, time_stamp
))usr_id
, EXTRACT(EPOCH FROM time_stamp)
, trans_id
))
usr_id
, time_stamp
, trans_id
)); it has the advantage of scanning the lives
table only once and, should you temporarily increase (if needed) work_mem to accommodate the sort in memory, it will be by far the fastest of all queries.All times above include retrieval of the full 10k rows result-set.
Your goal is minimal cost estimate and minimal query execution time, with an emphasis on estimated cost. Query execution can dependent significantly on runtime conditions (e.g. whether relevant rows are already fully cached in memory or not), whereas the cost estimate is not. On the other hand, keep in mind that cost estimate is exactly that, an estimate.
The best query execution time is obtained when running on a dedicated database without load (e.g. playing with pgAdminIII on a development PC.) Query time will vary in production based on actual machine load/data access spread. When one query appears slightly faster (<20%) than the other but has a much higher cost, it will generally be wiser to choose the one with higher execution time but lower cost.
When you expect that there will be no competition for memory on your production machine at the time the query is run (e.g. the RDBMS cache and filesystem cache won't be thrashed by concurrent queries and/or filesystem activity) then the query time you obtained in standalone (e.g. pgAdminIII on a development PC) mode will be representative. If there is contention on the production system, query time will degrade proportionally to the estimated cost ratio, as the query with the lower cost does not rely as much on cache whereas the query with higher cost will revisit the same data over and over (triggering additional I/O in the absence of a stable cache), e.g.:
cost | time (dedicated machine) | time (under load) |
-------------------+--------------------------+-----------------------+
some query A: 5k | (all data cached) 900ms | (less i/o) 1000ms |
some query B: 50k | (all data cached) 900ms | (lots of i/o) 10000ms |
Do not forget to run ANALYZE lives
once after creating the necessary indices.
Query #1
-- incrementally narrow down the result set via inner joins
-- the CBO may elect to perform one full index scan combined
-- with cascading index lookups, or as hash aggregates terminated
-- by one nested index lookup into lives - on my machine
-- the latter query plan was selected given my memory settings and
-- histogram
SELECT
l1.*
FROM
lives AS l1
INNER JOIN (
SELECT
usr_id,
MAX(time_stamp) AS time_stamp_max
FROM
lives
GROUP BY
usr_id
) AS l2
ON
l1.usr_id = l2.usr_id AND
l1.time_stamp = l2.time_stamp_max
INNER JOIN (
SELECT
usr_id,
time_stamp,
MAX(trans_id) AS trans_max
FROM
lives
GROUP BY
usr_id, time_stamp
) AS l3
ON
l1.usr_id = l3.usr_id AND
l1.time_stamp = l3.time_stamp AND
l1.trans_id = l3.trans_max
Query #2
-- cheat to obtain a max of the (time_stamp, trans_id) tuple in one pass
-- this results in a single table scan and one nested index lookup into lives,
-- by far the least I/O intensive operation even in case of great scarcity
-- of memory (least reliant on cache for the best performance)
SELECT
l1.*
FROM
lives AS l1
INNER JOIN (
SELECT
usr_id,
MAX(ARRAY[EXTRACT(EPOCH FROM time_stamp),trans_id])
AS compound_time_stamp
FROM
lives
GROUP BY
usr_id
) AS l2
ON
l1.usr_id = l2.usr_id AND
EXTRACT(EPOCH FROM l1.time_stamp) = l2.compound_time_stamp[1] AND
l1.trans_id = l2.compound_time_stamp[2]
2013/01/29 update
Finally, as of version 8.4, Postgres supports Window Function meaning you can write something as simple and efficient as:
Query #3
-- use Window Functions
-- performs a SINGLE scan of the table
SELECT DISTINCT ON (usr_id)
last_value(time_stamp) OVER wnd,
last_value(lives_remaining) OVER wnd,
usr_id,
last_value(trans_id) OVER wnd
FROM lives
WINDOW wnd AS (
PARTITION BY usr_id ORDER BY time_stamp, trans_id
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
);
There is a new option in Postgressql 9.5 called DISTINCT ON
SELECT DISTINCT ON (location) location, time, report
FROM weather_reports
ORDER BY location, time DESC;
It eliminates duplicate rows an leaves only the first row as defined my the ORDER BY clause.
see the official documentation
I would propose a clean version based on DISTINCT ON
(see docs):
SELECT DISTINCT ON (usr_id)
time_stamp,
lives_remaining,
usr_id,
trans_id
FROM lives
ORDER BY usr_id, time_stamp DESC, trans_id DESC;
SELECT l.*
FROM (
SELECT DISTINCT usr_id
FROM lives
) lo, lives l
WHERE l.ctid = (
SELECT ctid
FROM lives li
WHERE li.usr_id = lo.usr_id
ORDER BY
time_stamp DESC, trans_id DESC
LIMIT 1
)
Creating an index on (usr_id, time_stamp, trans_id)
will greatly improve this query.
You should always, always have some kind of PRIMARY KEY
in your tables.