There are table A and table B. I want to join these tables on two columns but only for selected rows of table A.
Query scenarios:
SELECT B.*
FROM B
INNER JOIN (SELECT * FROM A WHERE A.COLUMN1 BETWEEN somevalue1 AND somevalue2) C
ON B.COLUMN2 = C.COLUMN2
AND B.COLUMN3 = C.COLUMN3
OR
SELECT B.*
FROM B
INNER JOIN A
ON B.COLUMN2 = A.COLUMN2
AND B.COLUMN3 = A.COLUMN3
WHERE A.COLUMN1 BETWEEN somevalue1 AND somevalue2
Both tables A and B have millions of records. With WHERE condition table A will return me only 1000 results, so the actual join to be performed is to find matching details from B for only 1000 rows of A.
Query:
Which one should be faster? (I do not have access to view the query execution plan)
Thanks!
It's hard to predict performance here without actually measuring.
My instincts say the latter option should be faster because an optimizer may want to fully materialize the inner query before the join, which in addition to being slow all by itself could break any indexing that might help the join along. The optimizer for the latter option, on the other hand, should still be smart enough to pre-filter table A before the join, with no risk of breaking indexes and the ability only materialize results that match the join. Notice all the weasel words in there, though; my instincts could be way off in this case. The real lesson to take away from this is to measure your query using real data under conditions as close to actual as possible.
More importantly, I prefer the latter because (imo) it's just more readable and maintainable.
Related
I am trying to find a solution in order to improve the String searching process and I selected FULL-TEXT INDEX Strategy.
However, after implementing it, I still can see there is a performance hit when it comes to search by using multiple strings using multiple Full-Text Index tables with OR clauses.
(E.x. WHERE CONTAINS(F.*,'%Gayan%') OR CONTAINS(P.FirstName,'%John%'))
As a solution, I am trying to use CONTAINSTABLE expecting a performance improvement.
Now, I am facing an issue with CONTAINSTABLE when it comes to joining tables with a LEFT JOIN
Please go through the example below.
Query 1
SELECT F.Name,p.*
FROM P.Role PR
INNER JOIN P.Building F ON PR.PID = F.PID
LEFT JOIN CONTAINSTABLE(P.Building,*,'%John%') AS FFTIndex ON F.ID = FFTIndex.[Key]
LEFT JOIN P.Relationship PRSHIP ON PR.id = prship.ToRoleID
LEFT JOIN P.Role PR2 ON PRSHIP.ToRoleID = PR2.ID
LEFT JOIN P.Person p ON pr2.ID = p.PID
LEFT JOIN CONTAINSTABLE(P.Person,FirstName,'%John%') AS PFTIndex ON P.ID = PFTIndex.[Key]
WHERE F.Name IS NOT NULL
This produces the below result.
Query 2
SELECT F.Name,p.*
FROM P.Role PR
INNER JOIN P.Building F ON PR.PID = F.PID
INNER JOIN P.Relationship PRSHIP ON PR.id = prship.ToRoleID
INNER JOIN P.Role PR2 ON PRSHIP.ToRoleID = PR2.ID
INNER JOIN P.Person p ON pr2.ID = p.PID
WHERE CONTAINS(F.*,'%Gayan%') OR CONTAINS(P.FirstName,'%John%')
AND F.Name IS NOT NULL
Result
Expectation
To use query 1 in a way that works as the behavior of an SQL SERVER OR clause. As I can understand Query 1's CONTAINSTABLE, joins the data with the building table, and the rest of the results are going to ignore so that the CONTAINSTABLE of the Person table gets data that already contains the keyword filtered from the building table.
If the keyword = Building, I want to match the keyword in both the tables regardless of searching a saved record in both the tables. Having a record in each table is enough.
Summary
Query 2 performs well but is creates a slowness when the words in the indexes are growing. Query 1 seems optimized(When it comes to multiple online resources and MS Documentation),
however, it does not give me the expected output.
Is there any way to solve this problem?
I am not strictly attached to CONTAINSTABLE. Suggesting another optimization method will also be considerable.
Thank you.
Hard to say definitively without your full data set but a couple of options to explore
Remove Invalid % Wildcards
Why are you using '%SearchTerm%'? Does performance improve if you use the search term without the wildcards (%)? If you want a word that matches a prefix, try something like
WHERE CONTAINS (String,'"SearchTerm*"')
Try Temp Tables
My guess is CONTAINS is slightly faster than CONTAINSTABLE as it doesn't calculate a rank, but I don't know if anyone has ever attempted to benchmark it. Either way, I'd try saving off the matches to a temp table before joining up to the rest of the tables. This will allow the optimizer to create a better execution plan
SELECT ID INTO #Temp
FROM YourTable
WHERE CONTAINS (String,'"SearchTerm"')
SELECT *
FROM #Temp
INNER JOIN...
Optimize Full Text Index by Removing Noisy Words
You might find you have some noisy words aka words that reoccur many times in your data that are meaningless like "the" or perhaps some business jargon. Adding these to your stop list will mean your full text index will ignore them, making your index smaller thus faster
The query below will list indexed words with the most frequent at the top
Select *
From sys.dm_fts_index_keywords(Db_Id(),Object_Id('dbo.YourTable') /*Replace with your table name*/)
Order By document_count Desc
This OR That Criteria
For your WHERE CONTAINS(F.*,'%Gayan%') OR CONTAINS(P.FirstName,'%John%') criteria where you want this or that, is tricky. OR clauses generally perform even when using simple equality operators.
I'd try either doing two queries and union the results like:
SELECT * FROM Table1 F
/*Other joins and stuff*/
WHERE CONTAINS(F.*,'%Gayan%')
UNION
SELECT * FROM Table2 P
/*Other joins and stuff*/
WHERE CONTAINS(P.FirstName,'%John%')
OR this is much more work, but you could load all your data into giant denormalized table with all your columns. Then apply a full text index to that table and adjust your search criteria that way. It'd probably be the fastest method searching, but then you'd have to ensure the data is sync between the denormalized table and the underlying normalized tables
SELECT B.*,P.* INTO DenormalizedTable
FROM Building AS B
INNER JOIN People AS P
CREATE FULL TEXT INDEX ft ON DenormalizedTable
etc...
I have a somewhat complex view which includes a join to another view. For some reason the generated query plan is highly inefficient. The query runs for many hours. However if I select the sub-view into a temporary table first and then join with this, the same query finished in a few minutes.
My question is: Is there some kind of query hint or other trick which will force the optimizer to execute the joined sub-view in isolation before performing the join, just as when using a temp table? Clearly the default strategy chosen by the optimizer is not optimal.
I cannot use the temporary table-trick since views does not allow temporary tables. I understand I could probably rewrite everything to a stored procedure, but that would break composeability of views, and it seems also like bad for maintenance to rewrite everything just to trick the optimizer to not use a bad optimization.
Adam Machanic explained one such way at a SQL Saturday I recently attended. The presentation was called Clash of the Row Goals. The method involves using a TOP X at the beginning of the sub-select. He explained that when doing a TOP X, the query optimizer assumes it is more efficient to grab the TOP X rows one at a time. As long as you set X as a sufficiently large number (limit of INT or BIGINT?), the query will always get the correct results.
So one example that Adam provided:
SELECT
x.EmployeeId,
y.totalWorkers
FROM HumanResources.Employee AS x
INNER JOIN
(
SELECT
y0.ManagerId,
COUNT(*) AS totalWorkers
FROM HumanResources.Employee AS y0
GROUP BY
y0.ManagerId
) AS y ON
y.ManagerId = x.ManagerId
becomes:
SELECT
x.EmployeeId,
y.totalWorkers
FROM HumanResources.Employee AS x
INNER JOIN
(
SELECT TOP(2147483647)
y0.ManagerId,
COUNT(*) AS totalWorkers
FROM HumanResources.Employee AS y0
GROUP BY
y0.ManagerId
) AS y ON
y.ManagerId = x.ManagerId
It is a super cool trick and very useful.
When things get messy the query optimize often resorts to loop joins
If materializing to a temp fixed it then most likely that is the problem
The optimizer often does not deal with views very well
I would rewrite you view to not uses views
Join Hints (Transact-SQL)
You may be able to use these hints on views
Try merge and hash
Try changing the order of join
Move condition into the join whenever possible
select *
from table1
join table2
on table1.FK = table2.Key
where table2.desc = 'cat1'
should be
select *
from table1
join table2
on table1.FK = table2.Key
and table2.desc = 'cat1'
Now the query optimizer will get that correct but as the query gets more complex the query optimize goes into what I call stupid mode and loop joins. But that is also done to protect the server and have as little in memory as possible.
I have been trying to test this, but I have doubts about my tests as the timings vary so much.
-- Scenario 1
SELECT * FROM Foo f
INNER JOIN Bar b ON f.id = b.id
WHERE b.flag = true;
-- Scenario 2
SELECT * FROM Foo f
INNER JOIN Bar b ON b.flag = true AND f.id = b.id;
Logically it seems like scenario 2 would be more efficient, but I wasn't sure if SQL server is smart enough to optimize this or not.
Not sure why you think scenario 2 would "logically" be more efficient. On an INNER JOIN everything is basically a filter so SQL Server can collapse the logic to the exact same underlying plan shape. Here's an example from AdventureWorks2012 (click to enlarge):
I prefer separating the join criteria from the filter criteria, so will always write the query in the format on the left. However #HLGEM makes a good point, these clauses are interchangeable in this case only because it's an INNER JOIN. For an OUTER JOIN, it is very important to place the filters on the outer table in the join criteria, else you unwittingly end up with an INNER JOIN and drastically change the semantics of the query. So my advice about how the plan can be collapsed only holds true for inner joins.
If you're worried about performance, I'd start by getting rid of SELECT * and only pulling the columns you actually need (and make sure there's a covering index).
Four months later, another answer has emerged claiming that there usually will be a difference in performance, and that putting filter criteria in the ON clause will be better. While I won't dispute that it is certainly plausible that this could happen, I contend that it certainly isn't the norm and shouldn't be something you use as an excuse to always put all filter criteria in the ON clause.
The accepted answer is correct only for your test case.
An answer to the headline question as stated is yes, moving the constraint to the join condition can greatly improve the query and ensures. I have seen forms similar to this (but perhaps not exactly)...
select *
from A
inner join B
on B.id = a.id
inner join C
on C.id = A.id
where B.z = 1 and C.z = 2;
...not optimize to the same plan as the "on join" equivalents so I tend to use the "on join" constraints as a best practice even for the simpler cases that might have resolved optimally either way.
I have a dynamic query that runs indentifying CDs that members have not rented yet. I am using the NOT IN subquery but when I have large member table it makes them really slow. Any suggestions how to optimize the query
SELECT DVDTitle AS "DVD Title"
FROM DVD
WHERE DVDId NOT IN
(SELECT DISTINCT DVDId FROM Rental WHERE MemberId = AL240);
thanks
Using NOT EXISTS will have slightly better performance because it can "short circuit" rather than evaluating the entire set for each match. At the very least, it will be "no worse" than NOT IN or an OUTER JOIN, though there are exceptions to every rule. Here is how I would write this query:
SELECT DVDTitle AS [DVD Title]
FROM dbo.DVD AS d
WHERE NOT EXISTS
(
SELECT 1 FROM dbo.Rental
WHERE MemberId = 'AL240'
AND DVDId = d.DVDId
);
I would guess you will optimize performance better by investigating the execution plan and ensuring that your indexes are best suited for this query (without causing negative impact to other parts of your workload).
Also see Should I use NOT IN, OUTER APPLY, LEFT OUTER JOIN, EXCEPT, or NOT EXISTS?
SELECT DVDTitle AS "DVD Title"
FROM DVD d
left outer join Rental r on d.DVDId = r.DVDId
WHERE r.MemberId = 'AL240'
and r.DVDId is null
Make sure you have indexes on:
d.DVDId
r.DVDId
r.MemberId
I read this article:
Logical Processing Order of the SELECT statement
in end of article has been write ON and JOIN clause consider before WHERE.
Consider we have a master table that has 10 million records and a detail table (that has reference to master table(FK)) with 50 million record. We have a query that want just 100 record of detail table according a PK in master table.
In this situation ON and JOIN execute before WHERE?I mean that do we have 500 million record after JOIN and then WHERE apply to it?or first WHERE apply and then JOIN and ON Consider? If second answer is true do it has incoherence with top article?
thanks
In the case of an INNER JOIN or a table on the left in a LEFT JOIN, in many cases, the optimizer will find that it is better to perform any filtering first (highest selectivity) before actually performing whatever type of physical join - so there are obviously physical order of operations which are better.
To some extent you can sometimes control this (or interfere with this) with your SQL, for instance, with aggregates in subqueries.
The logical order of processing the constraints in the query can only be transformed according to known invariant transformations.
So:
SELECT *
FROM a
INNER JOIN b
ON a.id = b.id
WHERE a.something = something
AND b.something = something
is still logically equivalent to:
SELECT *
FROM a
INNER JOIN b
ON a.id = b.id
AND a.something = something
AND b.something = something
and they will generally have the same execution plan.
On the other hand:
SELECT *
FROM a
LEFT JOIN b
ON a.id = b.id
WHERE a.something = something
AND b.something = something
is NOT equivalent to:
SELECT *
FROM a
LEFT JOIN b
ON a.id = b.id
AND a.something = something
AND b.something = something
and so the optimizer isn't going to transform them into the same execution plan.
The optimizer is very smart and is able to move things around pretty successfully, including collapsing views and inline table-valued functions as well as even pushing things down through certain kinds of aggregates fairly successfully.
Typically, when you write SQL, it needs to be understandable, maintainable and correct. As far as efficiency in execution, if the optimizer is having difficulty turning the declarative SQL into an execution plan with acceptable performance, the code can sometimes be simplified or appropriate indexes or hints added or broken down into steps which should perform more quickly - all in successive orders of invasiveness.
It doesn't matter
Logical processing order is always honoured: regardless of actual processing order
INNER JOINs and WHERE conditions are effectively associative and commutative (hence the ANSI-89 "join in the where" syntax) so actual order doesn't matter
Logical order becomes important with outer joins and more complex queries: applying WHERE on an OUTER table changes the logic completely.
Again, it doesn't matter how the optimiser does it internally so long as the query semantics are maintained by following logical processing order.
And the key word here is "optimiser": it does exactly what it says
Just re-reading Paul White's excellent series on the Query Optimiser and remembered this question.
It is possible to use an undocumented command to disable specific transformation rules and get some insight into the transformations applied.
For (hopefully!) obvious reasons only try this on a development instance and remember to re-enable them and remove any suboptimal plans from the cache.
USE AdventureWorks2008;
/*Disable the rules*/
DBCC RULEOFF ('SELonJN');
DBCC RULEOFF ('BuildSpool');
SELECT P.ProductNumber,
P.ProductID,
I.Quantity
FROM Production.Product P
JOIN Production.ProductInventory I
ON I.ProductID = P.ProductID
WHERE I.ProductID < 3
OPTION (RECOMPILE)
You can see with those two rules disabled it does a cartesian join and filter after.
/*Re-enable them*/
DBCC RULEON ('SELonJN');
DBCC RULEON ('BuildSpool');
SELECT P.ProductNumber,
P.ProductID,
I.Quantity
FROM Production.Product P
JOIN Production.ProductInventory I
ON I.ProductID = P.ProductID
WHERE I.ProductID < 3
OPTION (RECOMPILE)
With them enabled the predicate is pushed right down into the index seek and so reduces the number of rows processed by the join operation.
There is no defined order. The SQL engine determines what order to perform the operations based on the execution strategy chosen by its optimizer.
I think you have misread ON as IN in the article.
However, the order it is showing in the article is correct (obviously it is msdn anyway). The ON and JOIN are executed before WHERE naturally because WHERE has to be applied as a filter on the temporary resultset obtained due to JOINS
The article just says it is logical order of execution and also at end of the paragraph it adds this line too ;)
"Note that the actual physical execution of the statement is determined by the query processor and the order may vary from this list."