Suppose I have a table of events with (indexed) columns id : uuid and created : timestamp.
The id column is unique, but the created column is not. I would like to walk the table in chronological order using the created column.
Something like this:
SELECT * FROM events WHERE created >= $<after> ORDER BY created ASC LIMIT 10
Here $<after> is a template parameter that is taken from the previous query.
Now, I can see two issues with this:
Since created is not unique, the order will not be fully defined. Perhaps the sort should be id, created?
Each row should only be on one page, but with this query the last row is always included on the next page.
How should I go about this in Postgres?
SELECT * FROM events
WHERE created >= $<after> and (id >= $<id> OR created > $<after>)
ORDER BY created ASC ,id ASC LIMIT 10
that way the events each timestamp values will be ordered by id. and you can split pages anywhere.
you can say the same thing this way:
SELECT * FROM events
WHERE (created,id) >= ($<after>,$<id>)
ORDER BY created ASC ,id ASC LIMIT 10
and for me this produces a slightly better plan.
An index on (created,id) will help performance most, but for
many circumstances an index on created may suffice.
First, as you said, you should enforce a total ordering. Since the main thing you care about is created, you should start with that. id could be the secondary ordering, a tie breaker invisible to the user that just ensures the ordering is consistent. Secondly, instead of messing around with conditions on created, you could just use an offset clause to return later results:
SELECT * FROM events ORDER BY created ASC, id ASC LIMIT 10 OFFSET <10 * page number>
-- Note that page number is zero based
I am a beginner in PostgreSQL and, after understanding very basic things, I want to find out how I can get a better performance (on a query) by using an index (one or more). I have read some documentation, but I would like a specific example so as to "catch" it.
MY EXAMPLE: Let's say I have just a table (MyTable) with three columns (Customer(text), Time(timestamp), Consumption(integer)) and I want to find the customer(s) with the maximum consumption on '2014-07-01 01:00:00'. MY SOLUTION (without index usage):
SELECT Customer FROM MyTable WHERE Time='2013-07-01 02:00:00'
AND Consumption=(SELECT MAX(consumption) FROM MyTable);
----> What would be the exact full code, using - at least one - index for the query-example above ?
The correct query (using a correlated subquery) would be:
SELECT Customer
FROM MyTable
WHERE Time = '2013-07-01 02:00:00' AND
Consumption = (SELECT MAX(t2.consumption) FROM MyTable t2 WHERE t2.Time = '2013-07-01 02:00:00');
The above is very reasonable. An alternative approach if you want exactly one row returned is:
SELECT Customer
FROM MyTable
WHERE Time = '2013-07-01 02:00:00'
ORDER BY Consumption DESC
LIMIT 1;
And the best index is MyTable(Time, Consumption, Customer).
Following is my query:
select * from table order by timestamp desc limit 10
this takes too much time compared to
select * from table limit 10
How can I optimize the first query to get to near performance of second query.
UPDATE: I don't have control over the db server, so can not index columns to gain performance.
Create an index on timestamp.
Quassnoi is correct -- you need an index on timestamp.
That said, if your timestamp field reasonably maps your primary key (e.g. a date_created or an invoice_date field), you can try this workaround:
select *
from (select * from table order by id desc limit 1000) as table
order by timestamp desc limit 10;
#Nishan is right. There is little you can do. If you do not need every column in the table you may gain a few milliseconds by explicitly asking for just the columns you need
How do you do LIMIT in DB2 for iSeries?
I have a table with more than 50,000 records and I want to return records 0 to 10,000, and records 10,000 to 20,000.
I know in SQL you write LIMIT 0,10000 at the end of the query for 0 to 10,000 and LIMIT 10000,10000 at the end of the query for 10000 to 20,000
So, how is this done in DB2? Whats the code and syntax?
(full query example is appreciated)
Using FETCH FIRST [n] ROWS ONLY:
http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.perf/db2z_fetchfirstnrows.htm
SELECT LASTNAME, FIRSTNAME, EMPNO, SALARY
FROM EMP
ORDER BY SALARY DESC
FETCH FIRST 20 ROWS ONLY;
To get ranges, you'd have to use ROW_NUMBER() (since v5r4) and use that within the WHERE clause: (stolen from here: http://www.justskins.com/forums/db2-select-how-to-123209.html)
SELECT code, name, address
FROM (
SELECT row_number() OVER ( ORDER BY code ) AS rid, code, name, address
FROM contacts
WHERE name LIKE '%Bob%'
) AS t
WHERE t.rid BETWEEN 20 AND 25;
Developed this method:
You NEED a table that has an unique value that can be ordered.
If you want rows 10,000 to 25,000 and your Table has 40,000 rows, first you need to get the starting point and total rows:
int start = 40000 - 10000;
int total = 25000 - 10000;
And then pass these by code to the query:
SELECT * FROM
(SELECT * FROM schema.mytable
ORDER BY userId DESC fetch first {start} rows only ) AS mini
ORDER BY mini.userId ASC fetch first {total} rows only
Support for OFFSET and LIMIT was recently added to DB2 for i 7.1 and 7.2. You need the following DB PTF group levels to get this support:
SF99702 level 9 for IBM i 7.2
SF99701 level 38 for IBM i 7.1
See here for more information: OFFSET and LIMIT documentation, DB2 for i Enhancement Wiki
Here's the solution I came up with:
select FIELD from TABLE where FIELD > LASTVAL order by FIELD fetch first N rows only;
By initializing LASTVAL to 0 (or '' for a text field), then setting it to the last value in the most recent set of records, this will step through the table in chunks of N records.
#elcool's solution is a smart idea, but you need to know total number of rows (which can even change while you are executing the query!). So I propose a modified version, which unfortunately needs 3 subqueries instead of 2:
select * from (
select * from (
select * from MYLIB.MYTABLE
order by MYID asc
fetch first {last} rows only
) I
order by MYID desc
fetch first {length} rows only
) II
order by MYID asc
where {last} should be replaced with row number of the last record I need and {length} should be replaced with the number of rows I need, calculated as last row - first row + 1.
E.g. if I want rows from 10 to 25 (totally 16 rows), {last} will be 25 and {length} will be 25-10+1=16.
Try this
SELECT * FROM
(
SELECT T.*, ROW_NUMBER() OVER() R FROM TABLE T
)
WHERE R BETWEEN 10000 AND 20000
The LIMIT clause allows you to limit the number of rows returned by the query. The LIMIT clause is an extension of the SELECT statement that has the following syntax:
SELECT select_list
FROM table_name
ORDER BY sort_expression
LIMIT n [OFFSET m];
In this syntax:
n is the number of rows to be returned.
m is the number of rows to skip before returning the n rows.
Another shorter version of LIMIT clause is as follows:
LIMIT m, n;
This syntax means skipping m rows and returning the next n rows from the result set.
A table may store rows in an unspecified order. If you don’t use the ORDER BY clause with the LIMIT clause, the returned rows are also unspecified. Therefore, it is a good practice to always use the ORDER BY clause with the LIMIT clause.
See Db2 LIMIT for more details.
You should also consider the OPTIMIZE FOR n ROWS clause. More details on all of this in the DB2 LUW documentation in the Guidelines for restricting SELECT statements topic:
The OPTIMIZE FOR clause declares the intent to retrieve only a subset of the result or to give priority to retrieving only the first few rows. The optimizer can then choose access plans that minimize the response time for retrieving the first few rows.
There are 2 solutions to paginate efficiently on a DB2 table :
1 - the technique using the function row_number() and the clause OVER which has been presented on another post ("SELECT row_number() OVER ( ORDER BY ... )"). On some big tables, I noticed sometimes a degradation of performances.
2 - the technique using a scrollable cursor. The implementation depends of the language used. That technique seems more robust on big tables.
I presented the 2 techniques implemented in PHP during a seminar next year. The slide is available on this link :
http://gregphplab.com/serendipity/uploads/slides/DB2_PHP_Best_practices.pdf
Sorry but this document is only in french.
Theres these available options:-
DB2 has several strategies to cope with this problem.
You can use the "scrollable cursor" in feature.
In this case you can open a cursor and, instead of re-issuing a query you can FETCH forward and backward.
This works great if your application can hold state since it doesn't require DB2 to rerun the query every time.
You can use the ROW_NUMBER() OLAP function to number rows and then return the subset you want.
This is ANSI SQL
You can use the ROWNUM pseudo columns which does the same as ROW_NUMBER() but is suitable if you have Oracle skills.
You can use LIMIT and OFFSET if you are more leaning to a mySQL or PostgreSQL dialect.
I have to fetch only 50 records at a time from database (DB2), for this I have been using Row_Number but now the persons are telling that this Row_Number is not stable and has bugs in it so now I have to write a different query for the same as I have to fetch only 50 records at a time.
So please can any body help me out for the same?
Thanks in advance.
The query which I have been using is
SELECT PLC.*
FROM
(SELECT
ROW_NUMBER() OVER (ORDER BY PRDLN_CTLG_OID) AS Row,
PRDLN_CTLG_OID,
PRODUCT_LINE_OID AS PRODUCT_LINE_OID,
RTRIM(CATALOG_ID) AS CATALOG_ID,
FROM
PROD_LINE_CATALOG
WHERE
PRODUCT_LINE_OID = :productLineOID AND ACTV_IND = 1
ORDER BY CATALOG_ID) PLC
WHERE
Row >= :startIndex AND Row <= :endIndex
ORDER BY
PLC.CATALOG_ID DESC
WITH UR
use the fetch clause
FETCH FIRST 50 ROWS ONLY
edit
It looks like you use a startIndex and endIndex value. I realize you compare these to Row, but do you have another index to compare? If not, and you insist on moving from Row_Number, maybe the table needs another index.
Select PLC.* From (Select omitting row_number())
Where yournewindex >= startIndex
Fetch first 50 rows only
Someone already answered that question. You are asking for pagination if I'm not wrong.
Fastest most/efficient way to do pagination with SQL searching DB2