Postgresql provides a parameter join_collapse_limit = 1 to disable order of joins. But when I set the parameter and reset the server, the query plan is not changed and there still is order join optimization. My query FROM table is like this
FROM
student as group_A,
student as group_B,
student as group_C
WHERE ...
If users want to use join_collapse_limit = 1 to disable join reordering, explicit JOINS must be provided in the query. For example, the above FROM table should be
FROM
student as group_A CROSS JOIN
student as group_B CROSS JOIN
student as group_C
WHERE ...
Related
This is a current postgres query I have:
sql = """
SELECT
vms.campaign_id,
avg(vms.open_rate_uplift) as open_rate_average,
avg(vms.click_rate_uplift) as click_rate_average,
avg(vms.conversion_rate_uplift) as conversion_rate_average,
avg(cms.incremental_opens),
avg(cms.incremental_clicks),
avg(cms.incremental_conversions)
FROM
experiments.variant_metric_snapshot vms
INNER JOIN experiments.campaign_metric_snapshot cms ON vms.campaign_id = cms.campaign_id
WHERE
vms.campaign_id IN %(campaign_ids)s
GROUP BY
vms.campaign_id
"""
whereby I get the average incremental_opens, incremental_clicks, and incremental_conversions per campaign group from the cms table. However, instead of the average, I want the most recent values for the 3 fields. See the cms table screenshot below - I want the values from the record with the greatest (i.e. most recent) event_id (instead of an average for all records) for a given group).
How can I do this? Thanks
It sounds like you want a lateral join.
FROM
experiments.variant_metric_snapshot vms
CROSS JOIN LATERAL (select * from experiments.campaign_metric_snapshot cms where vms.campaign_id = cms.campaign_id order by event_id desc LIMIT 1) cms
WHERE...
If you are after a quick and dirty solution you can use array_agg function with minimal change to your query.
SELECT
vms.campaign_id,
avg(vms.open_rate_uplift) as open_rate_average,
avg(vms.click_rate_uplift) as click_rate_average,
avg(vms.conversion_rate_uplift) as conversion_rate_average,
(array_agg(cms.incremental_opens ORDER BY cms.event_id DESC))[1] AS incremental_opens,
..
FROM
experiments.variant_metric_snapshot vms
INNER JOIN experiments.campaign_metric_snapshot cms ON vms.campaign_id = cms.campaign_id
WHERE
vms.campaign_id IN %(campaign_ids)s
GROUP BY
vms.campaign_id;
Postgres allows you to create a table using inheritance. We have a design where we have 1400 tables that inherit from one main table. These tables are for each of our vendor's inventory.
When I want to query stock for a vendor, I just query the main table. When running Explain, the explanation says that it is going through all 1400 indexes and quite a few of the inherited tables. This causes the query to run very slowly. If I query only the vendor's stock table, I cut the query time to less than 50% of the time by querying the main table.
We have a join on another table that pulls identifiers for the vendor's partner vendors and we also want to query their stock. Example:
SELECT
(select m2.company from sup.members m2 where m2.id = u.id) as company,
u.id,
u.item,
DATE_PART('day', CURRENT_TIMESTAMP - u.datein::timestamp) AS daysinstock,
u.grade as condition,
u.stockno AS stocknumber,
u.ic,
CASE WHEN u.rprice > 0 THEN
u.rprice
ELSE
NULL
END AS price,
u.qty
FROM pub.net u
LEFT JOIN sup.members m1
ON m1.id = u.id OR u.id = any(regexp_split_to_array(m1.partnerslist,','))
WHERE u.ic in ('01036') -- part to query
AND m1.id = 'N40' -- vendor to query
The n40_stock table has stock for the vendor with id = N40 and N40's partner vendors (partnerslist) are G01, G06, G21, K17, N49, V02, M16 so I would also want
to query the g01_stock, g06_stock, g21_stock, k17_stock, n49_stock, v02_stock, and m16_stock tables.
I know about the ONLY clause but is there away to modify this query to get the data from ONLY the specific inherited tables?
Edit
This decreases the time to under 800ms, but I'd like it less:
WITH cte as (
SELECT partnerslist as a FROM sup.members WHERE id = 'N40'
)
SELECT
(select m2.company from sup.members m2 where m2.id = u.id) as company,
u.id,
u.item,
DATE_PART('day', CURRENT_TIMESTAMP - u.datein::timestamp) AS daysinstock,
u.grade as condition,
u.stockno AS stocknumber,
u.ic,
CASE WHEN u.rprice > 0 THEN
u.rprice
ELSE
NULL
END AS price,
u.qty
FROM pub.net u
WHERE u.ic in ('01036') -- part to query
AND u.id = any(regexp_split_to_array('N40,'||(select a from cte), ','))
I cannot retrieve the company from sup.members in the cte because I need the one from the u.id, which is different when the partner changes in the where clause.
Inherited table lookups are based on the actual WHERE clause, which maps to the CHECK table constraint. Simply inheriting tables is not good enough.
https://www.postgresql.org/docs/9.6/static/ddl-partitioning.html
Caveat, you can not use a dynamically created variables where the actual value is not implemented in the raw query. This results in a check of all inherited tables.
Okay. So the question that I got asked by the teacher was this:
(5 marks) Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition. (E.g. find the titles of films rented by a particular customer.) Note the hints on the course news page if your query returns nothing.
Here is the layout of the database im working with:
http://www.postgresqltutorial.com/wp-content/uploads/2013/05/PostgreSQL-Sample-Database.png
The hint to us was this:
PostgreSQL hint:
If a natural join doesn't produce any results in the dvdrental DB, it is because many tables have the last update: timestamp field, and thus the natural join tries to join on that field as well as the intended field.
e.g.
select *
from film natural join inventory;
does not work because of this - it produces an empty table (no results).
Instead, use
select *
from film, inventory
where film.film_id = inventory.film_id;
This is what I did:
select *
from film, customer
where film.film_id = customer.customer_id;
The problem is I cannot get a particular customer.
I tried doing customer_id = 2; but it returns a error.
Really need help!
Well, it seems that you would like to join two tables that have no direct relation with each other, there's your issue:
where film.film_id = customer.customer_id
To find which films are rented by which customer you would have to join customer table with rental, then with inventory and finally with film.
The task description states
Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition.quote
Assum I have three tables User, Product and UserProduct. Once a user buys a product a record in the UserProduct table is created.
I would like to calculate an average number of products being bought by users. I can do this with plain SQL on mySql RDBMS like as follows:
select avg(c) from (select count(*) as c from user_product up join user u on (up.user_id = u.id) group by u.id) as res
Any chance I can do the same with JPA?
Update: I know that subqueries may be used in the WHERE or HAVING clause only, but still...
(I don't think I have titled this question correctly - but I don't know how to describe it)
Here is what I am trying to do:
Let's say I have a Person table that has a PersonID field. And let's say that a Person can belong to many Groups. So there is a Group table with a GroupID field and a GroupMembership table that is a many-to-many join between the two tables and the GroupMembership table has a PersonID field and a GroupID field. So far, it is a simple many to many join.
Given a list of GroupIDs I would like to be able to write a query that returns all of the people that are in ALL of those groups (not any one of those groups). And the query should be able to handle any number of GroupIDs. I would like to avoid dynamic SQL.
Is there some simple way of doing this that I am missing?
Thanks,
Corey
select person_id, count(*) from groupmembership
where group_id in ([your list of group ids])
group by person_id
having count(*) = [size of your list of group ids]
Edited: thank you dotjoe!
Basically you are looking for Persons for whom there is no group he is not a member of, so
select *
from Person p
where not exists (
select 1
from Group g
where not exists (
select 1
from GroupMembership gm
where gm.PersonID = p.ID
and gm.GroupID = g.ID
)
)
You're basically not going to avoid "dynamic" SQL in the sense of dynamically generating the query at query time. There's no way to hand a list around in SQL (well, there is, table variables, but getting them into the system from C# is either impossible (2005 & below) or else annoying (2008)).
One way that you could do it with multiple queries is to insert your list into a work table (probably a process-keyed table) and join against that table. The only other option would be to use a dynamic query such as the ones specified by Jonathan and hongliang.