The executed sql:
select last() from stable group by tbname limit 1,120;
a supertable with 4000 sub-tables when I tested it.
When I executed this sql in the terminal, it did not return any data.
When I change the sql to select last() from stable group by tbname limit **0,**120;
4000 rows of data are returned.
When using the following, it also returns 4000 rows of data
select last(*) from stable group by tbname limit 10;
In TDengine database，you can use "slimit" to limit the output for group by function but not "limit" ,"limit" is for normal query”
Related
Since SnowFlake is a columnar database, does it impact performance when you use COUNT(*) vs COUNT(column)? And this is assuming that the column that you're referencing does NOT have any NULLs
As a_horse_with_no_name explained these two functions are different but you already mentioned that the column has no NULL values. So they should return the same result in your case.
More important thing is, Snowflake has a special optimization for the COUNT function. As far I see, it does NOT impact performance if you use COUNT(*) or COUNT(column), even when the column contains NULL values! For both of them, Snowflake uses METADATA statistics, so it does not actually count rows.
You can test it with SNOWFLAKE_SAMPLE_DATA:
select count(*) from snowflake_sample_data.TPCH_SF1000.LINEITEM;
-- 5999989709
select count(L_ORDERKEY) from snowflake_sample_data.TPCH_SF1000.LINEITEM;
-- 5999989709
Both queries will return a result immediately although the table size is about 170G, and contain more than 5G rows.
I have to add this extra information because of the conversation between Niru and a_horse_with_no_name. a_horse_with_no_name said:
Even if all columns of a row are NULL, count(*) should include that row in the result. If it doesn't this is a clear violation of the SQL standard
I'm not sure about the SQL standard but when you use COUNT(*), Snowflake doesn't check if the columns are NULL or not (as you expected). I can see why Niru misunderstood the documents, the docs and the samples should be improved.
If you run my sample queries, you will see that they are completed in milliseconds. We are talking about counting almost 6 billion rows:
select count(*) from snowflake_sample_data.TPCH_SF1000.LINEITEM;
-- completes in milliseconds
select count(L_ORDERKEY) from snowflake_sample_data.TPCH_SF1000.LINEITEM;
-- completes in milliseconds
But if I do a little modification on the query, it takes about 3 minutes on the same warehouse (XSMALL):
select count(t.*) from sample_data.TPCH_SF1000.LINEITEM t;
-- completes in 3 minutes!?
Here is the trick:
Alias.*, which indicates that the function should return the number of rows that do not contain any NULLs.
https://docs.snowflake.com/en/sql-reference/functions/count.html#arguments
Only if you use alias.* (like I used t.* in my sample), Snowflake will check if all columns are null when producing the count. This is why it is much slower, and this is why there shouldn't be any performance issues when you are running COUNT(XYZ) or COUNT(*) on a table.
Here is the snowflake doc.. hope it helps
https://docs.snowflake.com/en/sql-reference/functions/count.html Please refer to snowflake document.. it does effect count(alias.*) will check the each column in the row where as count(column) do null check only on that column..
How can I LIMIT the result returned by a query in Adaptive Server IQ/12.5.0/0306?
The following gives me a generic error near LIMIT:
SELECT * FROM mytable LIMIT 10, 10;
Any idea why? This is my first time with this dbms
Sybase IQ uses row_count to limit the number of rows returned.
You will have to set the row count at the beginning of your statement and decide whether the statement should be TEMPORARY or not.
ROW_COUNT
SET options
LIMIT statement is not supported in Sybase IQ 12. I think there is not simple or clean solution, similarly like in old SQL server. But there are some approches that works for SQL server 2000 and should work also for Sybase IQ 12. I don't promise that queries below will work on copy&paste.
Subquery
SELECT TOP 10 *
FROM mytable
WHERE Id NOT IN (
SELECT TOP 10 Id FROM mytable ORDER BY OrderingColumn
)
ORDER BY OrderingColumn
Basically, it fetches 10 rows but also skips first 10 rows. To get this works, rows must be unique and ordering is important. Id cannot be more times in results. Otherwise you can filter out valid rows.
Asc-Desc
Another workaround depends on ordering. It uses ordering and fetches 10 rows for second page and you have to take care of last page (it does not work properly with simple formula page * rows per page).
SELECT *
FROM
(
SELECT TOP 10 *
FROM
(
SELECT TOP 20 * -- (page * rows per page)
FROM mytable
ORDER BY Id
) AS t1
ORDER BY Id DESC
) AS t2
ORDER BY Id ASC
I've found some info about non working subqueries in FROM statement in ASE 12. This approach maybe is not possible.
Basic iteration
In this scenario you can just iterate through rows. Let's assume id of tenth row is 15. Then it will select next 10 rows after tenth row. Bad things happen when you will order by another column than Id. It is not possible.
SELECT TOP 10 *
FROM mytable
WHERE Id > 15
ORDER BY Id
Here is article about another workarounds in SQL server 2000. Some should also works in similar ways in Sybase IQ 12.
http://www.codeproject.com/Articles/6936/Paging-of-Large-Resultsets-in-ASP-NET
All those things are workarounds. If you can try to migrate on newer version.
I'm using SQL Server 2012 and I was wondering if there is a way to select, for example, the third result from a query. I have looked into LIMIT and OFFSET but I'm not 100% sure if it applies to SQL Server, however I've been told that there is something to do what I want in SQL Server 2012.
SELECT *
FROM YourTable
ORDER BY OrderingColumn ASC
OFFSET 2 ROWS /*Skip first 2 rows*/
FETCH NEXT 1 ROWS ONLY
Note: You cannot use OFFSET ... FETCH without doing an ORDER BY first
I would recommend
select * from table ORDER BY OrderingColumn ASC LIMIT n,1
This is a quirk of limit where if you give it a range, it only returns that range. It works in MySQL too.
Can’t you just select the second index? This is the way i would take say the third post in my table.
//index 3
$index3 = DB::query(‘SELECT * from posts’)[2];
Then you will have an array that is ready to go for you.
I have a table in sql 2005 with a big count of data - smth like 1 500 000 rows right now and later it should be more. Before paging I need to detect what rows the user can read (sql query for checking is a heavy which refer to several other tables) and the result should be paged.
What the best practice to work with the huge table that should be filtered and paged after all?
Thanks in advance!
If you want to return paginated results in SQL Server your best bet is probably to use the ROW_NUMBER() function. Here is an example that would get you the 400th-410th results:
SELECT ID, Name, Date
FROM (SELECT TOP 410 ROW_NUMBER() OVER (ORDER BY id)
AS Row, ID, Name, Date FROM MyTable)
AS MyPagedTable
WHERE Row >= 400 AND Row <= 410
Make sure you have the proper indexes in place. If you are getting performance issues then I would recommend looking at the execution plan and seeing where the problem areas are.
In Sql Server 2008, many options are available for database paging via stored procedure. For example, see here and here.
OPTIONS:
ROW_NUMBER() function
ROWCOUNT
CURSORS
temporary tables
Nested SQL queries
OTHERS
Paging using ROW_NUMBER() is known to have performance issues:
Please advise, which paging method has the best performance (for large tables with JOINs) ?
Please also provide links to relevant article(s), if possible.
Thank You.
One question you have to answer is if you want to display the total number of rows to the end user. To calculate the number of the last page, you also need the last row number.
If you can do without that information, a temporary table is a good option. You can select the pirmary key and use LIMIT to retrieve keys up to the key you're interested in. If you do this right, the typical use case will only retrieve the first few pages.
If you need the last page number, you can use ROW_NUMBER(). Using a temporary table won't be much faster because you can't use the LIMIT clause, making this strategy the equivalent of a ROW_NUMBER() calculation.
We can get a rowcount using following query.
WITH data AS
(
SELECT ROW_NUMBER() OVER (order by memberid ) AS rowid, memberid
FROM Customer
)
SELECT *, (select count(*) from data) AS TotalCount
FROM data
WHERE rowid > 20 AND rowid <= 30