in Oracle SQL, there is a possible criteria called rownum. Can i confirm that rownum will be executed at last as just a limit for number of records return?
or could it be executed first, before other WHERE SQL criteria (let's if we put rownum prior to the others)?
It's not the equivalent of LIMIT in other languages. If you plan on limiting the number of records with rownum, you'll need to subquery the ORDER BY on the inside and use rownum in the outer query. Order of elements in your WHERE clause does not matter.
See this excellent article by Tom Kyte.
Yes, within a WHERE clause, ROWNUM is always evaluated last, after all other predicates have been evaluated, regardless of their order.
It is evaluated before any GROUP BY, or ORDER BY clauses, however.
Related
I'm trying to write query that's not using offset (because as I just have learnt offset fetches all data which causes performance overhead). with ROW_NUMBER window function. For instance:
SELECT id
FROM(
SELECT id, ROW_NUMBER() over (order by id) rn
FROM users) sq
WHERE rn > 1000
Does it require all rows to be fetched as it would be with offset 1000? I mean, does it make a sense to use such query instead of
SELECT if
FROM users
OFFSET 1000
? Do I get performance improvement on large amount of data?
Check out the window function docs. Window functions operate on the result set, after the fetch:
Window functions are permitted only in the SELECT list and the ORDER
BY clause of the query. They are forbidden elsewhere, such as in GROUP
BY, HAVING and WHERE clauses. This is because they logically execute
after the processing of those clauses. Also, window functions execute
after regular aggregate functions. This means it is valid to include
an aggregate function call in the arguments of a window function, but
not vice versa.
Does it make sense to use the row_number() query? Well, it produces the same result set. However, the query basically has to assign row_number() to all the rows in order to find the ones that meet the requirement.
The second query, however, is lacking an order by. When using offset, you should have an order by:
SELECT id
FROM users u
ORDER BY id
OFFSET 1000
I would imagine that this is more efficient than using row_number(), but actual timings would demonstrate that.
what does the line (rowid,0) mean in the following query
select * from emp
WHERE (ROWID,0) in (
select rowid, mod(rownum,2) from emp
);
i dont get the line WHERE (ROWID,0).
what is it?
thanx in advance
IN clause in Oracle SQL can support column groups. You can do things like this:
select ...
from tab1
where (tab1.col1, tab1.col2) in (
select tab2.refcol1, tab2.refcol2
from tab2
)
That can be useful in many cases.
In your particular case, the subquery use for the second expression mod(rownum,2). Since there is no order by, that means that rownum will be in whichever order the database retrieves the rows - that might be a full table scan or a fast full index scan.
Then by using mod every other row in the subquery gets the value 0, every other row gets the value 1.
The IN clause then filters on second value in the subquery being equal to 0. The end result is that this query retrieves half of your employees. Which half will depend on which access path the optimizer chooses.
Not sure what dialect of sql you're using, but it appears that since the subquery in the IN clause has two columns in the select list, then the (ROWID,0) indicates which columns align with the subquery. I have never seen multiple columns in an IN statment's select list before.
This is a syntax used by some databases (but not all) that allows you to do in with multiple values.
With in, this is the same as:
where exists (select 1
from emp e2
where e2.rowid = emp.rowid and
mod(rownum, 2) = 0
)
I should note that if you are using Oracle (which allows this syntax), then you are using rownum in a subquery with no order by. The results are going to be rather arbitrary. However, the intention seems to be to return every other row, in some sense.
According to this link, when using rownum in a query, it is called in the following order of operations.
The FROM/WHERE clause goes first.
ROWNUM is assigned and incremented to each output row from the FROM/WHERE clause.
SELECT is applied.
GROUP BY is applied.
HAVING is applied.
ORDER BY is applied.
I want to know where the AND would be categorized on this list. Would it be evaluated at the same time as the WHERE? What if the WHERE has a rownum and the AND does not?
The AND has no role in this. When result set is being constructed, the rownum is assigned to the results before outermost ordering. Filtering on ROWNUM is a hard stop from feeding results up from deeper in the execution plan. Therefore for example a construct like where rownum > 5 returns no rows.
Hopefully this helps. If not, please elaborate in your question and/or explain why you are asking. There are alternatives that are sometimes better, such as row_number().
I just had an update not work that had where field_value is not null and rownum = 1.
since rownum = 1 was for a row that had field_value = null that was the row it tired to return and then no action occurred in the update.
So with in the Where clause there is also an order of operation.
need to put () around it or make it a sub query to be sure rownum is on the results of the other parts of the where cause.
I used a distinct instead of rownum = 1 for my case.
I'm working on a query on the SEDE:
select top 20
row_number() over(order by "percentage approved" desc, approved desc),
row_number() over(order by "total edits" asc),
*
from editors
where "total edits" > 30
What is the ordering of the result set, taking into account the two window functions?
I suspect it's undefined but couldn't find a definitive answer. OTOH, results from queries with one such window function were ordered according to the over(order by ...) clause.
The results can be returned in any order.
Now, they will often be returned in the same order as specified in the OVER clause, but this is just because SQL Server is likely to pick a query plan that sorts the rows to calculate the aggregate. This is by no means guaranteed, as it could pick a different query plan at any time, especially as you make your query more complex which extends the space of possible query plans.
The result set of ANY SQL Server query that doesn't have an explicit ORDER BY is undefined.
This includes when you have window functions within the query, or an ORDER BY in a subquery. The result order will depend on a lot of factors, none of which are guaranteed unless you specify an ORDER BY.
I have following SQL statement.
SELECT t.client_id,max(t.points) AS "max" FROM sessions GROUP BY t.client_id;
It simply lists client id's with maximum amount of points they've achieved. Now I want to sort the results by max(t.points). Normally I would use ORDER BY, but I have no idea how to use it with groups. I know using value from SELECT list is prohibited in following clauses, so adding ORDER BY max at the end of query won't work.
How can I sort those results after grouping, then?
Best regards
SELECT t.client_id, max(t.points) AS "max"
FROM sessions t
GROUP BY t.client_id
order by max(t.points) desc
It is not quite correct that values from the SELECT list are prohibited in following clauses. In fact, ORDER BY is logically processed after the SELECT list and can refer to SELECT list result names (in contrast with GROUP BY). So the normal way to write your query would be
SELECT t.client_id, max(t.points) AS "max"
FROM sessions
GROUP BY t.client_id
ORDER BY max;
This way of expressing it is SQL-92 and should be very portable. The other way to do it is by column number, e.g.,
ORDER BY 2;
These are the only two ways to do this in SQL-92.
SQL:1999 and later also allow referring to arbitrary expressions in the sort list, so you could just do ORDER BY max(t.points), but that's clearly more cumbersome, and possibly less portable. The ordering by column number was removed in SQL:1999, so it's technically no longer standard, but probably still widely supported.
Since you have tagged as Postgres: Postgres allows a non-standard GROUP BY and ORDER BY column number. So you could have
SELECT t.client_id, max(t.points) AS "max"
FROM sessions t
GROUP BY 1
order by 2 desc
After parsing, this is identical to RedFilter’s solution.