So I have this query (and have encountered or coded a bunch of similar ones during my life :) ) which is extremely inefficient in terms of performance, due to the subquery.
I'm running pgsql currently but have had this issue with mysql and mssql as well.
Sometimes I can use MAX() but here I have 2 different columns: runners.id (the one I need to find my data) and runners.updated_at (the one on which I could do MAX()).
Any tips?
SELECT
ROUND(CAST(AVG(DATE_PART('day', current_claim_event.updated_at - claims.created_at)) AS NUMERIC),1)
AS average, count(*)
FROM claims_events current_claim_event
INNER JOIN claims ON claims.id = current_claim_event.claim_id
WHERE current_claim_event.id = (
SELECT runners.id
FROM claims_events runners
WHERE runners.claim_id = current_claim_event.claim_id
ORDER BY runners.updated_at DESC
LIMIT 1
);
Related
I'm stuck in an (apparently) extremely trivial task that I can't make work , and I really feel no chance than to ask for advice.
I used to deal with PHP/MySQL more than 10 years ago and I might be quite rusty now that I'm dealing with an SQLite DB using Qt5.
Basically I'm selecting some records while wanting to make some math operations on the fetched columns. I recall (and re-read some documentation and examples) that the keyword "AS" is going to conveniently rename (alias) a value.
So for example I have this query, where "X" is an integer number that I render into this big Qt string before executing it with a QSqlQuery. This query lets me select all the electronic components used in a Project and calculate how many of them to order (rounding to the nearest multiple of 5) and the total price per component.
SELECT Inventory.id, UsedItems.pid, UsedItems.RefDes, Inventory.name, Inventory.category,
Inventory.type, Inventory.package, Inventory.value, Inventory.manufacturer,
Inventory.price, UsedItems.qty_used as used_qty,
UsedItems.qty_used*X AS To_Order,
ROUND((UsedItems.qty_used*X/5)+0.5)*5*CAST((X > 0) AS INT) AS Nearest5,
Inventory.price*Nearest5 AS TotPrice
FROM Inventory
LEFT JOIN UsedItems ON Inventory.id=UsedItems.cid
WHERE UsedItems.pid='1'
ORDER BY RefDes, value ASC
So, for example, I aliased UsedItems.qty_used as used_qty. At first I tried to use it in the next field, multiplying it by X, writing "used_qty*X AS To_Order" ... Query failed. Well, no worries, I had just put the original tab.field name and it worked.
Going further, I have a complex calculation and I want to use its result on the next field, but the same issue popped out: if I alias "ROUND(...)" AS Nearest5, and then try to use this value by multiplying it in the next field, the query will fail.
Please note: the query WORKS, but ONLY if I don't use aliases in the following fields, namely if I don't use the alias Nearest5 in the TotPrice field. I just want to avoid re-writing the whole ROUND(...) thing for the TotPrice field.
What am I missing/doing wrong? Either SQLite does not support aliases on the same query or I am using a wrong syntax and I am just too stuck/confused to see the mistake (which I'm sure it has to be really stupid).
Column aliases defined in a SELECT cannot be used:
For other expressions in the same SELECT.
For filtering in the WHERE.
For conditions in the FROM clause.
Many databases also restrict their use in GROUP BY and HAVING.
All databases support them in ORDER BY.
This is how SQL works. The issue is two things:
The logic order of processing clauses in the query (i.e. how they are compiled). This affects the scoping of parameters.
The order of processing expressions in the SELECT. This is indeterminate. There is no requirement for the ordering of parameters.
For a simple example, what should x refer to in this example?
select x as a, y as x
from t
where x = 2;
By not allowing duplicates, SQL engines do not have to make a choice. The value is always t.x.
You can try with nested queries.
A SELECT query can be nested in another SELECT query within the FROM clause;
multiple queries can be nested, for example by following the following pattern:
SELECT *,[your last Expression] AS LastExp From (SELECT *,[your Middle Expression] AS MidExp FROM (SELECT *,[your first Expression] AS FirstExp FROM yourTables));
Obviously, respecting the order that the expressions of the innermost select query can be used by subsequent select queries:
the first expressions can be used by all other queries, but the other intermediate expressions can only be used by queries that are further upstream.
For your case, your query may be:
SELECT *, PRC*Nearest5 AS TotPrice FROM (SELECT *, ROUND((UsedItems.qty_used*X/5)+0.5)*5*CAST((X > 0) AS INT) AS Nearest5 FROM (SELECT Inventory.id, UsedItems.pid, UsedItems.RefDes, Inventory.name, Inventory.category, Inventory.type, Inventory.package, Inventory.value, Inventory.manufacturer, Inventory.price AS PRC, UsedItems.qty_used*X AS To_Order FROM Inventory LEFT JOIN UsedItems ON Inventory.id=UsedItems.cid WHERE UsedItems.pid='1' ORDER BY RefDes, value ASC))
So I am honestly a little puzzled by this!
I have a query that returns a set of transactions that contain both repair costs and an odometer reading at the time of repair on the master level. To get an accurate Cost per mile reading I need to do a subquery to get both the first meter reading between a starting date and an end date, and an ending meter.
(select top 1 wf2.ro_num
from wotrans wotr2
left join wofile wf2
on wotr2.rop_ro_num = wf2.ro_num
and wotr2.rop_fac = wf2.ro_fac
where wotr.rop_veh_num = wotr2.rop_veh_num
and wotr.rop_veh_facility = wotr2.rop_veh_facility
AND ((#sdate = '01/01/1900 00:00:00' and wotr2.rop_tran_date = 0)
OR ([dbo].[udf_RTA_ConvertDateInt](#sdate) <= wotr2.rop_tran_date
AND [dbo].[udf_RTA_ConvertDateInt](#edate) >= wotr2.rop_tran_date))
order by wotr2.rop_tran_date asc) as highMeter
The reason I have the tables aliased as xx2 is because those tables are also used in the main query, and I don't want these to interact with each other except to pull the correct vehicle number and facility.
Basically when I run the main query it returns a value that is not correct; it returns the one that is second(keep in mind that the first and second have the same date.) But when I take the subquery and just copy and paste it into it's own query and run it, it returns the correct value.
I do have a work around for this, but I am just curious as to why this happening. I have searched quite a bit and found not much(other than the fact that people don't like order bys in subqueries). Talking to one of my friends that also does quite a bit of SQL scripting, it looks to us as if the subquery is ordering differently than the subquery by itsself when you have multiple values that are the same for the order by(i.e. 10 dates of 08/05/2016).
Any ideas would be helpful!
Like I said I have a work around that works in this one case, but don't know yet if it will work on a larger dataset.
Let me know if you want more code.
I have a table order, which is very straightforward, it is storing order data.
I have a view, which is storing currency pair and currency rate. The view is created as below:
create or replace view view_currency_rate as (
select c.* from currency_rate c, (
select curr_from, curr_to, max(rate_date) max_rate_date from currency_rate
where system_rate > 0
group by curr_from, curr_to) r
where c.curr_from = r.curr_from
and c.curr_to = r.curr_to
and c.rate_date = r.max_rate_date
and c.system_rate > 0
);
nothing fancy here, this view populate the latest currency rate (curr_from -> curr_to) from the currency_rate table.
When I do as below, it populate 80k row (all data) because I have plenty of records in order table. And the time spent is less than 5 seconds.
First Query:
select * from
VIEW_CURRENCY_RATE c, order a
where
c.curr_from = A.CURRENCY;
I want to add in more filter, so I thought it could be faster, so I added this:
Second Query:
select * from
VIEW_CURRENCY_RATE c, order a
where
a.id = 'xxxx'
and c.curr_from = A.CURRENCY;
And now it run over 1 minute! I totally have no idea what happen to this. I thought it would be some oracle optimizer goes wrong, so I try to find another way, think of just the 80K data can be populated quite fast, so I try to get the data from it, so I nested the SQL as below:
select * from (
select * from
VIEW_CURRENCY_RATE c, order a
where
c.curr_from = A.CURRENCY
)
where id = 'xxxx';
It run damn slow as well! I running out of idea, can anyone explain what happen to my script?
Updated on 6-Sep-2016
After I know how to 'explain plan', I capture the screen:
Fist query (fast one with 80K data):
Second query (slow one):
The slow one totally break the view and form a new SQL! This is super weird that how can Oracle optimize this like that?
It seems problem relates to the plan of second query. because it uses of nest loops inplace of hash joint.
at first check if _hash_join_enable is true if it isn't true change it to true. if it is true there are some problem with oracle optimizer. for test it use of USE_HASH(tab2 tab1) hint.
Regards
mohsen
I am using Mike solution, I re-write the script, and it is running fast now, although the root cause is not determined, probably due to the oracle optimizer algorithm working in different way that I expect.
iam not a big ORACLE - SQL Expert, so i hope someone knows a good way to find the "duplicate" record wich is causing the: single-row subquery returns more than one row error.
This my Statement:
SELECT
CAST(af.SAP_SID AS VARCHAR2(4000)) APP_ID,
(SELECT DR_OPTION
FROM
DR_OPTIONS
WHERE DR_OPTIONS.ID = (
select dr_option from applications where applications.sap_sid = af.sap_sid)) DR_OPTION
FROM
APPLICATIONS_FILER_VIEW af
it works on my test system, so iam "sure" there must be an error inside the available data records, but i have no idea how to find those ..
Try with this query:
select applications.sap_sid, count(dr_option)
from applications
group by applications.sap_sid
having count(dr_option) > 1
This should give you the sap_sid of the duplicated rows
I'd suggest simplifying your query:
SELECT CAST(af.SAP_SID AS VARCHAR2(4000)) APP_ID,
dr.DR_OPTION
FROM APPLICATIONS_FILER_VIEW af
INNER JOIN applications a ON af.sap_sid = a.sap_sid
INNER JOIN DR_OPTIONS dr ON a.dr_option = dr.ID
I would investigate what you get when you run:
select dr_option from applications where applications.sap_sid = af.sap_sid
but you could force only one row to be returned (I see this as being a fudge and would not recommend using it at least add an order by to have some control over the row being returned) with something like:
SELECT
CAST(af.SAP_SID AS VARCHAR2(4000)) APP_ID,
(SELECT DR_OPTION
FROM
DR_OPTIONS
WHERE DR_OPTIONS.ID = (
select dr_option
from applications
where applications.sap_sid = af.sap_sid
and rownumber = 1)
) DR_OPTION
FROM
APPLICATIONS_FILER_VIEW af
(not tested just googled how to limit results in oracle)
If you fix the data issue (as per A.B.Cades comment) then I would recommend converting it to use joins as per weenoid's answer. this would also highlight other data issues that may arise in the future.
IN SHORT: I have never fixed anything in this way.. the real answer is to investigate the multiple rows returned and decide what you want to do maybe:
add more where clauses
order the results and only select top row
actually keep the duplicates as they represent a scenario you have not thought of before
Hey guys. This is a follow-on from this question:
After getting the right data and making some tweaks based on requests from business, I've now got this mini-beast on my hands. This query should return the total number of new jobseeker registrations and the number of new uploaded CV's:
SELECT COUNT(j.jobseeker_id) as new_registrations,
(
SELECT
COUNT(c.cv_id)
FROM
tb_cv as c, tb_jobseeker, tb_industry
WHERE
UNIX_TIMESTAMP(c.created_at) >= '1241125200'
AND
UNIX_TIMESTAMP(c.created_at) <= '1243717200'
AND
tb_jobseeker.industry_id = tb_industry.industry_id
)
AS uploaded_cvs
FROM
tb_jobseeker as j, tb_industry as i
WHERE
j.created_at BETWEEN '2009-05-01' AND '2009-05-31'
AND
i.industry_id = j.industry_id
GROUP BY i.description, MONTH(j.created_at)
Notes:
- The two values in the UNIX TIMESTAMP functions are passed in as parameters from the report module in our backend.
Every time I run it, MySQL chokes and lingers silently into the ether of the Interweb.
Help is appreciated.
Update: Hey guys. Thanks a lot for all the thoughtful and helpful comments. I'm only 2 weeks into my role here, so I'm still learning the schema. So, this query is somewhere between a thumbsuck and an educated guess. Will start to answer all your questions now.
tb_cv is not connected to the other tables in the sub-query. I guess this is the root cause for the slow query. It causes generation of a Cartesian product, yielding a lot more rows than you probably need.
Other than that I'd say you need indexes on tb_jobseeker.created_at, tb_cv.created_at and tb_industry.industry_id, and you might want to get rid of the UNIX_TIMESTAMP() calls in the sub-query since they prevent use of an index. Use BETWEEN and the actual field values instead.
Here is my attempt at understanding your query and writing a better version. I guess you want to get the count of new jobseeker registrations and new uploaded CVs per month per industry:
SELECT
i.industry_id,
i.description,
MONTH(j.created_at) AS month_created,
YEAR(j.created_at) AS year_created,
COUNT(DISTINCT j.jobseeker_id) AS new_registrations,
COUNT(cv.cv_id) AS uploaded_cvs
FROM
tb_cv AS cv
INNER JOIN tb_jobseeker AS j ON j.jobseeker_id = cv.jobseeker_id
INNER JOIN tb_industry AS i ON i.industry_id = j.industry_id
WHERE
j.created_at BETWEEN '2009-05-01' AND '2009-05-31'
AND cv.created_at BETWEEN '2009-05-01' AND '2009-05-31'
GROUP BY
i.industry_id,
i.description,
MONTH(j.created_at),
YEAR(j.created_at)
A few things I noticed while writing the query:
you GROUP BY values you don't output in the end. Why? (I've added the grouped field to the output list.)
you JOIN three tables in the sub-query while only ever using values from one of them. Why? I don't see what it would be good for, other than filtering out CV records that don't have a jobseeker or an industry attached — which I find hard to imagine. (I've removed the entire sub-query and used a simple COUNT instead.)
Your sub-query returns the same value every time. Did you maybe mean to correlate it in some way, to the industry maybe?.
The sub-query runs once for every record in a grouped query without being wrapped in an aggregate function.
First and foremost it may be worth moving the 'UNIX_TIMESTAMP' conversions to the other side of the equation (that is, perform a reverse function on the literal timestamp values at the other side of the >= and <=). That'll avoid the inner query having to perform the conversions for every record, rather than once for the query.
Also, why does the uploaded_cvs query not have any where clause linking it to the outer query? Am I missing something here?