Maybe someone can explain this to me, but when querying a data table from Oracle, where multiple records exist for a key (say a customer ID), the record that appears first for that customer can vary if there is no implicit "order by" statement enforcing the order by say an alternate field such as a transaction type. So running the same query on the same table could yield a different record ordering than from 10 minutes ago.
E.g., one run could yield:
Cust_ID, Transaction_Type
123 A
123 B
Unless an "order by Transaction_Type" clause is used, Oracle could arbitrarily return the following result the next time the query is run:
Cust_ID, Transaction_Type
123 B
123 A
I guess I was under the impression that there was a database default ordering of rows in Oracle which (perhaps) reflected the physical ordering on the disk medium. In other words, an arbitrary order that is immutable and would guarantee the same result when a query is rerun.
Does this have to do with the optimizer and how it decides where to most efficiently retrieve the data?
Of course the best practice from a programming perspective is to force whatever ordering is required, I was just a little unsettled by this behavior.
The order of rows returned to the application from a SELECT statement is COMPLETELY ARBITRARY unless otherwise specified. If you want, need, or expect rows to return in a certain order, it is the user's responsibility to specify such an order.
(Caveat: Some versions of Oracle would implicitly sort data in ascending order if certain operations were used, such as DISTINCT, UNION, MINUS, INTERSECT, or GROUP BY. However, as Oracle has implemented hash sorting, the nature of the sort of the data can vary, and lots of SQL relying on that feature broke.)
There is no default ordering, ever. If you don't specify ORDER BY, you can get the same result the first 10000 times, then it can change.
Note that this is also true even with ORDER BY for equal values. For example:
Col1 Col2
1 1
2 1
3 2
4 2
If you use ORDER BY Col2, you still don't know if row 1 or 2 will come first.
Just image the rows in a table like balls in a basket. Do the balls have an order?
I dont't think there is any DBMS that guarantees an order if ORDER BY is not specified.
Some might always return the rows in the order they were inserted, but that is an implementation side effect.
Some execution plans might cause the result set to be ordered even without an ORDER BY, but again this is an implementation side-effect that you should not rely on.
If an ORDER BY clause is not present the database (not just Oracle - any relational database) is free to return rows in whatever order it happens to find them. This will vary depending on the query plan chosen by the optimizer.
If the order in which the rows are returned matters you must use an ORDER BY clause. You may sometimes get lucky and the rows will come back in the order you want them to be even without an ORDER BY, but there is no guarantee that A) you will get lucky on other queries, and B) the order in which the rows are returned tomorrow will be the same as the order in which they're returned today.
In addition, updates to the database product may change the behavior of queries. We had to scramble a bit when doing a major version upgrade last year when we found that Oracle 10 returned GROUP BY results in a different order than did Oracle 9. Reason - no ORDER BY clause.
ORDER BY - when the order of the returned data really matters.
The simple answer is that the SQL standard says that there is no default order for queries that do not have an ORDER BY statement, so you should never assume one.
The real reason would probably relate to the hashes assigned to each row as it is pulled into the record set. There is no reason to assume consistent hashing.
if you don't use ORDER BY, the order is arbitrary; however, dependent on phisical storage and memory aspects.
so, if you repeat the same query hundreds of times in 10 minutes, you will get almost the same order everytime, because probably nothing changes.
Things that could change the "noorder order" are:
the executing plan - if is changed(you have pointed
that)
inserts and deletes on the tables involved in the query.
other things like presence in memory of the rows.(other querys on other tables could influence that)
When you get into parallel data retrieval I/O isn't it possible to get different sequences on different runs, even with no change to the stored data?
That is, in a multiprocessing environment the order of completion of parallel threads is undefined and can vary with what else is happening on the same shared processor.
As I'm new to Oracle database engine, I noticed this behavior in my SELECT statements that has no ORDER BY.
I've been using Microsoft SQL Server for years now. SQL Server Engine always will retrieve data ordered by the table's "Clustered Index" which is basically the Primary Key Index. SQL Server will always insert new data in a sequential order based on the clustered index.
So when you perform a select on a table without order by in SQL Server, it will always retrieve data ordered by primary key value.
ORDER BY can cause serious performance overhead, that's why you do not want to use it unless you are not happy with inconsistent results order.
I ended up with a conclusion that in ALL my Oracle queries I must use ORDER BY or I will end up with unpredicted order which will greatly effect my end-user reports.
Related
Suppose, if following rows are inserted in chronological order into a table:
row1, row2, row3, row4, ..., row1000, row1001.
After a while, we delete/remove the latest row1001.
As in this post: How to get Top 5 records in SqLite?
If the below command is run:
SELECT * FROM <table> LIMIT 1;
Will it assuredly provide the "row1000"?
If no, then is there any efficient way to get the latest row(s)
without traversing through all the rows? -- i.e. without using
combination of ORDER BY and DESC.
[Note: For now I am using "SQLite", but it will be interesting for me to know about SQL in general as well.]
You're misunderstanding how SQL works. You're thinking row-by-row which is wrong. SQL does not "traverse rows" as per your concern; it operates on data as "sets".
Others have pointed out that relational database cannot be assumed to have any particular ordering, so you must use ORDER BY to explicitly specify ordering.
However (not mentioned yet is that), in order to ensure it performs efficiently, you need to create an appropriate index.
Whether you have an index or not, the correct query is:
SELECT <cols>
FROM <table>
ORDER BY <sort-cols> [DESC] LIMIT <no-rows>
Note that if you don't have an index the database will load all data and probably sort in memory to find the TOP n.
If you do have the appropriate index, the database will use the best index available to retrieve the TOP n rows as efficiently as possible.
Note that the sqllite documentation is very clear on the matter. The section on ORDER BY explains that ordering is undefined. And nothing in the section on LIMIT contradicts this (it simply constrains the number of rows returned).
If a SELECT statement that returns more than one row does not have an ORDER BY clause, the order in which the rows are returned is undefined.
This behaviour is also consistent with the ANSI standard and all major SQL implementations. Note that any database vendor that guaranteed any kind of ordering would have to sacrifice performance to the detriment of queries trying to retrieve data but not caring about order. (Not good for business.)
As a side note, flawed assumptions about ordering is an easy mistake to make (similar to flawed assumptions about uninitialised local variables).
RDBMS implementations are very likely to make ordering appear consistent. They follow a certain algorithm for adding data, a certain algorithm for retrieving data. And as a result, their operations are highly repeatable (it's what we love (and hate) about computers). So things repeatably look the same.
Theoretical examples:
Inserting a row results in the row being added to the next available free space. So data appears sequential. But an update would have to move the row to a new location if it no longer fits.
The DB engine might retrieve data sequentially from clustered index pages and seem to use clustered index as the 'natural ordering' ... until one day a page-split puts one of the pages in a different location. * Or a new version of the DMBS might cache certain data for performance, and suddenly order changes.
Real-world example:
The MS SQL Server 6.5 implementation of GROUP BY had the side-effect of also sorting by the group-by columns. When MS (in version 7 or 2000) implemented some performance improvements, GROUP BY would by default, return data in a hashed order. Many people blamed MS for breaking their queries when in fact they had made false assumptions and failed to ORDER BY their results as needed.
This is why the only guarantee of a specific ordering is to use the ORDER BY clause.
No. Table records have no inherent order. So it is undefined which row(s) to get with a LIMIT clause without an ORDER BY.
SQLite in its current implemantation may return the latest inserted row, but even if that were the case you must not rely on it.
Give a table a datetime column or some sortkey, if record order is important for you.
In SQL, data is stored in tables unordered. What comes out first one day might not be the same the next.
ORDER BY, or some other specific selection criteria is required to guarantee the correct value.
I need to run a query that groups the result and orders it. When I used the following query I noticed that the results were ordered by the field name:
SELECT name, count(name)
FROM contacts
GROUP BY name
HAVING count(name)>1
Originally I planed on using the following query:
SELECT name, count(name)
FROM contacts
GROUP BY name
HAVING count(name)>1
ORDER BY name
I'm worried that order by significantly slows the running time.
Can I depend on ms-access to always order by the field I am grouping by, and eliminate the order by?
EDIT: I tried grouping different fields in other tables and it was always ordered by the grouped field.
I have found answers to this question to other SQL DBMSs, but not access.
How GROUP BY and ORDER BY work in general
Databases usually choose between sorting and hashing when creating groups for GROUP BY or DISTINCT operations. If they do choose sorting, you might get lucky and the sorting is stable between the application of GROUP BY and the actual result set consumption. But at some later point, this may break as the database might suddenly prefer an order-less hashing algorithm to produce groups.
In no database, you should ever rely on any implicit ordering behaviour. You should always use explicit ORDER BY. If the database is sophisticated enough, adding an explicit ORDER BY clause will hint that sorting is more optimal for the grouping operation as well, as the sorting can then be re-used in the query execution pipeline.
How this translates to your observation
I tried grouping different fields in other tables and it was always ordered by the grouped field.
Have you exhaustively tried all possible queries that could ever be expressed? I.e. have you tried:
JOIN
OUTER JOIN
semi-JOIN (using EXISTS or IN)
anti-JOIN (using NOT EXISTS or NOT IN)
filtering
grouping by many many columns
DISTINCT + GROUP BY (this will certainly break your ordering)
UNION or UNION ALL (which defeats this argument anyway)
I bet you haven't. And even if you tried all of the above, can you be sure there isn't a very peculiar configuration where the above breaks, just because you've observed the behaviour in some (many) experiments?
You cannot.
MS Access specific behaviour
As far as MS Access is concerned, consider the documentation on ORDER BY
Remarks
ORDER BY is optional. However, if you want your data displayed in sorted order, then you must use ORDER BY.
Notice the wording. "You must use ORDER BY". So, MS Acces is no different from other databases.
The answer
So your question about performance is going in the wrong direction. You cannot sacrifice correctness for performance in this case. Better tackle performance by using indexes.
Here is the MSDN documentation for the GROUP BY clause in Access SQL:
https://msdn.microsoft.com/en-us/library/bb177905(v=office.12).aspx
The page makes no reference to any implied or automatic ordering of results - if you do see desired ordering without an explicit ORDER BY then it is entirely coincidental.
The only way to guarantee the particular ordering of results in SQL is with ORDER BY.
There is a slight performance problem with using ORDER BY (in general) in that it requires the DBMS to get all of the results first before it outputs the first row of results (though the DBMS is free to use an "online sort" algorithm that sorts data as it gets each row from its backing store, it still needs to get the last row from the backing store before it can return the first row to the client (in case the last row from the backing-store happens to be the 1st result according to the ORDER BY) - however unless you're querying tens of thousands of rows in a latency-sensitive application this is not a problem - and as you're using Access already it's very clear that this is not a performance-sensitive application.
I have read it over and over again that SQL, at its heart, is an unordered model. That means executing the same SQL query multiple times can return result-set in different order, unless there's an "order by" clause included. Can someone explain why can a SQL query return result-set in different order in different instances of running the query? It may not be the case always, but its certainly possible.
Algorithmically speaking, does query plan not play any role in determining the order of result-set when there is no "order by" clause? I mean when there is query plan for some query, how does the algorithm not always return data in the same order?
Note: Am not questioning the use of order by, am asking why there is no-guarantee, as in, am trying to understand the challenges due to which there cannot be any guarantee.
Some SQL Server examples where the exact same execution plan can return differently ordered results are
An unordered index scan might be carried out in either allocation order or key order dependant on the isolation level in effect.
The merry go round scanning feature allows scans to be shared between concurrent queries.
Parallel plans are often non deterministic and order of results might depend on the degree of parallelism selected at runtime and concurrent workload on the server.
If the plan has nested loops with unordered prefetch this allows the inner side of the join to proceed using data from whichever I/Os happened to complete first
Martin Smith has some great examples, but the absolute dead simple way to demonstrate when SQL Server will change the plan used (and therefore the ordering that a query without ORDER BY will be used, based on the different plan) is to add a covering index. Take this simple example:
CREATE TABLE dbo.floob
(
blat INT PRIMARY KEY,
x VARCHAR(32)
);
INSERT dbo.floob VALUES(1,'zzz'),(2,'aaa'),(3,'mmm');
This will order by the clustered PK:
SELECT x FROM dbo.floob;
Results:
x
----
zzz
aaa
mmm
Now, let's add an index that happens to cover the query above.
CREATE INDEX x ON dbo.floob(x);
The index causes a recompile of the above query when we run it again; now it orders by the new index, because that index provides a more efficient way for SQL Server to return the results to satisfy the query:
SELECT x FROM dbo.floob;
Results:
x
----
aaa
mmm
zzz
Take a look at the plans - neither has a sort operator, they are just - without any other ordering input - relying on the inherent order of the index, and they are scanning the whole index because they have to (and the cheapest way for SQL Server to scan the index is in order). (Of course even in these simple cases, some of the factors in Martin's answer could influence a different order; but this holds true in the absence of any of those factors.)
As others have stated, the ONLY WAY TO RELY ON ORDER is to SPECIFY AN ORDER BY. Please write that down somewhere. It doesn't matter how many scenarios exist where this belief can break; the fact that there is even one makes it futile to try to find some guidelines for when you can be lazy and not use an ORDER BY clause. Just use it, always, or be prepared for the data to not always come back in the same order.
Some related thoughts on this:
Bad habits to kick : relying on undocumented behavior
Why people think some SQL Server 2000 behaviors live on… 12 years later
Quote from Wikipedia:
"As SQL is a declarative programming language, SELECT queries specify a result set, but do not specify how to calculate it. The database translates the query into a "query plan" which may vary between executions, database versions and database software. This functionality is called the "query optimizer" as it is responsible for finding the best possible execution plan for the query, within applicable constraints."
It all depends on what the query optimizer picks as a plan - table scan, index scan, index seek, etc.
Other factors that might influence picking a plan are table/index statistics and parameter sniffing to name a few.
In short, the order is never guaranteed without an ORDER BY clause.
It's simple: if you need the data ordered then use an ORDER BY. It's not hard!
It may not cause you a problem today or next week or even next month but one day it will.
I've been on a project where we needed to rewrite dozens (or maybe hundreds) of queries after an upgrade to Oracle 10g caused GROUP BY to be evaluated in a different way than in had on Oracle 9i, meaning that the queries weren't necessarily ordered by the grouped columns anymore. Not fun and simple to avoid.
Remember that SQL is declarative language so you are telling the DBMS what you want and the DBMS is then working out how to get it. It will bring back the same results every time but may evaluate in a different way each time: there are no guarantees.
Just one simple example of where this might cause you problems is that new rows appear at the end of the table if you select from the table.... until they don't because you've deleted some rows and the DBMS decides to fill in the empty space.
There are an unknowable number of ways it can go wrong unless you use ORDER BY.
Why does water boil at 100 degrees C? Because that's the way it's defined.
Why are there no guarantees about result ordering without an ORDER BY? Because that's the way it's defined.
The DBMS will probably use the same query plan the next time and that query plan will probably return the data in the same order: but that is not a guarantee, not even close to a guarantee.
If you don't specify an ORDER BY then the order will depend on the plan it uses, for example if the query did a table scan and used no index then the result would be the "natural order" or the order of the PK. However if the plan determines to use IndexA that is based on columnA then the order would be in that order. Make sense?
We are using SQL Server 2008. We have a table called response which has a primary key called response_id. It also has a column called bid_id. When we execute the query
‘select * from response where bid_id = x’
without an ‘order by’ we are getting results in mostly ascending order (default), but once in a while in descending order (very random). Is it possible in sql that the same sql query might return a resultset in different order if executed several times without order by? We used to have SQL Server 2000 till 5 months ago and never faced this problem. Does SQL Server 2008 deal differently with sql queries without ‘order by’?
Thanks.
In any SQL statement, on any database, you need to define an ORDER BY clause to guarantee the order is consistent.
Do you mean this happens for varying values of x?
As OMG Ponies says the order is not guaranteed unless you specify one.
In terms of an explanation perhaps you are getting a clustered index scan when it thinks there will be a significant portion of rows matching the bid_id = x criteria, hence them being in order of response_id, but it is looking them up via a non clustered index on the bid_id column when it expects fewer records to match the condition. These different access strategies could lead to them being returned in different orders. Adding an ORDER BY would cause it to favour a strategy that will leave them sorted or it would add an additional sort step to the plan before the results get returned.
Always use ORDER BY if order is important.
What is cached in memory as opposed to what is on disk will affect the order in which the SQL Server engine gets the rows, thus affecting the order of the rows if ORDER BY is not used.
The following URL has an interesting, if not directly related, discussion on the topic of the cache.
http://blog.sqlauthority.com/2010/06/17/sql-server-data-pages-in-buffer-pool-data-stored-in-memory-cache/
This is a very common question. I wrote up a canned answer:
Without ORDER BY, there is no default sort order.
Kind of a whimsical question, always something I've wondered about and I figure knowing why it does what it does might deepen my understanding a bit.
Let's say I do "SELECT TOP 10 * FROM TableName". In short timeframes, the same 10 rows come back, so it doesn't seem random. They weren't the first or last created. In my massive sample size of...one table, it isn't returning the min or max auto-incrementing primary key value.
I also figure the problem gets more complex when taking joins into account.
My database of choice is MSSQL, but I figure this might be an interesting question regardless of the platform.
If you do not supply an ORDER BY clause on a SELECT statement you will get rows back in arbitrary order.
The actual order is undefined, and depends on which blocks/records are already cached in memory, what order I/O is performed in, when threads in the database server are scheduled to run, and so on.
There's no rhyme or reason to the order and you should never base any expectations on what order rows will be in unless you supply an ORDER BY.
If they're not ordered by the calling query, I believe they're just returned in the order they were read off disk. This may vary because of the types of joins used or the indexes that looked up the values.
You can see this if the table has a clustered index on it (and you're just selecting - a JOIN can re-order things) - a SELECT will return the rows in clustered-index-order, even without an ORDER BY clause.
There is a very detailed explanation with examples here: http://sqlserverpedia.com/blog/sql-server-bloggers/its-the-natural-order-of-things-not/
"How do database servers decide which order to return rows without any “order by” statements?"
They simply do not take any "decision" with respect to ordering. They see the user doesn't care about ordering, and so they don't care either. And thus they simply go out to find the requested rows. The order in which they find them is normally the order in which you get them. That order depends on user-unpredictable things like the chosen physical access paths, ordering of physical records inside the database's physical files, etc. etc.
Don't let yourself be misled by the ordering as you get it, in the case where you didn't explicitly specify an ordering in your query. If you don't specify an ordering in your query, no ordering in the result set is guaranteed, even if in practice results seem to suggest that some ordering appears to be adhered to by the server.