I am beginner with indexes. I want to create index for this SQL expression which takes too much time to execute so I would like on what exact columns should I create index?
I am using DB2 db but never mind I think that question is very general.
My SQL expression is:
select * from incident where (relatedtoglobal=1)
and globalticketid in (select ticketid from INCIDENT where status='RESOLVED')
and statusdate <='2012-10-09 12:12:12'
Should I create index with this 5 columns or how?
Thanks
Your query:
select *
from incident
where relatedtoglobal = 1
and globalticketid in ( select ticketid
from INCIDENT
where status='RESOLVED'
)
and statusdate <='2012-10-09 12:12:12' ;
And the subquery inside:
select ticketid
from INCIDENT
where status='RESOLVED'
An index on (status, ticketid) will certainly help efficiency of the subquery evaluation and thus of the query.
For the query, besides the previous index, you'll need one more index. The (relatedtoglobal, globalticketid) may be sufficient.
I'm not sure if a more complex indexing would/could be used by the DB2 engine.
Like one on (relatedtoglobal, globalticketid) INCLUDE (statusdate) or
Two indices, one on (relatedtoglobal, globalticketid) and one on (relatedtoglobal, statusdate)
The DB2 documentation is not an easy read but has many details. Start with CREATE INDEX statement and Implementing Indexes.
Related
when I'm trying to use this query in oracle it's taking 0.04054s and while using the same query in PostgreSQL then it taking 49.8min how can I change the query to increase performance in PostgreSQL?
SELECT
"ID","IMAGE","TITLE","SERVICE_DESC"
,"STATUS", "ACTION","REMOVAL_TEXT","SERVICE_PROVIDER"
, "SERVICE_PROVIDER_NAME"
FROM (
SELECT DISTINCT "ID","IMAGE"
,"TITLE", "SERVICE_DESC"
, COALESCE("STATUS",'N') as "STATUS"
,"ACTION","REMOVAL_TEXT","CREATED_DT"
,"SERVICE_PROVIDER", "SERVICE_PROVIDER_NAME"
FROM MZP_ADP.ALL_SERVICE_DETAILS
WHERE "ZIP_CODE"='55005' AND "MAKE_LIVE" = 'Y'
AND "LOCATION_ID" = '2407605'
AND "END_DATE" > CURRENT_TIMESTAMP(0)::TIMESTAMP WITHOUT TIME ZONE
AND "IS_ACTIVE" = 'Y' order by "CREATED_DT" desc
) alias;
There can be a lot of problem. (rowcounts, hardwer, no index)
First, what is the rowcounts of table?
Have you inserted a lot of row some time before?
(Then can be REINDEX TABLE TABLE_NAME , And VACUUM ANALYZE TABLE_NAME help.)
CHECK indexes on this columns
LOCATION_ID
ZIP_CODE
CREATED_DT
END_DATE
Why is the select in subselect?
Please eliminate.
Can you eliminate the distinct with an additional where clause?
Please Share plans and rowcount than can we say more details.
EXPLAIN ANALIZE SELECT..
You can try:
Create this index
create index ALL_SERVICE_DETAILS_CMP_INDEX on MZP_ADP.ALL_SERVICE_DETAILS ("ZIP_CODE", "MAKE_LIVE", "LOCATION_ID", "END_DATE", "IS_ACTIVE");
Remove parent select
Remove distinct (if there is at least one unique column in the select)
Apply few things for performance boosting
VACUUM FULL for tables (it also rebuild indexes). But any confusion execute rebuild indexes
VACUUM (FULL, ANALYZE) table_name;
REINDEX TABLE table_name;
Increase work_mem and maintenance_work_mem as per your memory and server
configuration
Use GROUP BY instead of DISTINCT (distinct is slower)
Remove ORDER BY inside subquery. If needed then use it outside
create an composite index with column ZIP_CODE, LOCATION_ID, END_DATE and use
proper ordering in WHERE clause (As MAKE_LIVE and IS_ACTIVE are flag type so need
to add first in index)
EXPLAIN ANALYZE QUERY for checking execution time and using proper index in query
Pseudocode:
SELECT columns
FROM (SELECT columns
FROM table
WHERE searching columns as per index creation
GROUP BY WITHOUT aggregated COLUMNS) t
ORDER BY columns -- if needed
I have an SQL query as simple as:
select * from recent_cases where user_id=1000000 and case_id=10095;
It takes up to 0.4 seconds to execute it in Oracle. And when I do 20 requests in a row, it takes > 10s.
The table 'recent_cases' has 4 columns: ID, USER_ID, CASE_ID and VISITED_DATE. Currently there are only 38 records in this table.
Also, there are 3 indexes on this table: on ID column, on USER_ID column, and on (USER_ID, CASE_ID) columns pair.
Any ideas?
One theory -- the table has a very large data segment and high water mark near the end, but the statistics are not prompting the optimiser to use an index. Therefore you're getting a slow full table scan. You could ALTER TABLE ... MOVE and rebuild the indexes to fix such a problem, or COALESCE it.
Oracle Databases have a function called "analyze table". This function can speed up select statements a lot, even if there are just a few rows in the table.
Here are some links which might help you:
http://www.dba-oracle.com/t_oracle_analyze_table.htm
http://docs.oracle.com/cd/B28359_01/server.111/b28310/general002.htm
I have table in Oracle database which is called my_table for example. It is type of log table. It has an incremental column which is named "id" and "registration_number" which is unique for registered users. Now I want to get latest changes for registered users so I wrote queries below to accomplish this task:
First version:
SELECT t.*
FROM my_table t
WHERE t.id =
(SELECT MAX(id) FROM my_table t_m WHERE t_m.registration_number = t.registration_number
);
Second version:
SELECT t.*
FROM my_table t
INNER JOIN
( SELECT MAX(id) m_id FROM my_table GROUP BY registration_number
) t_m
ON t.id = t_m.m_id;
My first question is which of above queries is recommended and why? And second one is if sometimes there is about 70.000 insert to this table but mostly the number of inserted rows is changing between 0 and 2000 is it reasonable to add index to this table?
An analytical query might be the fastest way to get the latest change for each registered user:
SELECT registration_number, id
FROM (
SELECT
registration_number,
id,
ROW_NUMBER() OVER (PARTITION BY registration_number ORDER BY id DESC) AS IDRankByUser
FROM my_table
)
WHERE IDRankByUser = 1
As for indexes, I'm assuming you already have an index by registration_number. An additional index on id will help the query, but maybe not by much and maybe not enough to justify the index. I say that because if you're inserting 70K rows at one time the additional index will slow down the INSERT. You'll have to experiment (and check the execution plans) to figure out if the index is worth it.
In order to check for faster query, you should check the execution plan and cost and it will give you a fair idea. But i agree with solution of Ed Gibbs as analytics make query run much faster.
If you feel this table is going to grow very big then i would suggest partitioning the table and using local indexes. They will definitely help you to form faster queries.
In cases where you want to insert lots of rows then indexes slow down insertion as with each insertion index also has to be updated[I will not recommend index on ID]. There are 2 solutions i have think of for this:
You can drop index before insertion and then recreate it after insertion.
Use reverse key indexes. Check this link : http://oracletoday.blogspot.in/2006/09/there-is-option-to-create-index.html. Reverse key index can impact your query a bit so there will be trade off.
If you look for faster solution and there is a really need to maintain list of last activity for each user, then most robust solution is to maintain separate table with unique registration_number values and rowid of last record created in log table.
E.g. (only for demo, not checked for syntax validity, sequences and triggers omitted):
create table my_log(id number not null, registration_number number, action_id varchar2(100))
/
create table last_user_action(refgistration_number number not null, last_action rowid)
/
alter table last_user_action
add constraint pk_last_user_action primary key (registration_number) using index
/
create or replace procedure write_log(p_reg_num number, p_action_id varchar2)
is
v_row_id rowid;
begin
insert into my_log(registration_number, action_id)
values(p_reg_num, p_action_id)
returning rowid into v_row_id;
update last_user_action
set last_action = v_row_id
where registration_number = p_reg_num;
end;
/
With such schema you can simple query last actions for every user with good performance:
select
from
last_user_action lua,
my_log l
where
l.rowid (+) = lua.last_action
Rowid is physical storage identity directly addressing storage block and you can't use it after moving to another server, restoring from backups etc. But if you need such functionality it's simple to add id column from my_log table to last_user_action too, and use one or another depending on requirements.
I have the very table in sql server 2008 with lot of data
|ID|Name|Column_1|Column_2|
|..|....|........|........|
more than 18,000 records. So i need to the the row with the lowest value of Column_1 that is date but could by any data type (that is unsorted) so I use these sentence
SELECT TOP 1 ID, Name from table ORDER BY Column_1 ASC
But this is very very slow. And i think that i don't need to to sort the whole table. My question es how to get the same date with out using TOP 1 and ORDER BY
I cannot see why 18,000 rows of information would cause too much of a slow down, but that is obviously without seeing what the data is you are storing.
If you are regularly going to be using the Column_1 field, then I would suggest you place a non-clustered index on it... that will speed up your query.
You can do it by "designing" your table via Sql Server Management Studio, or directly via TSQL...
CREATE INDEX IX_myTable_Column_1 ON myTable (Column_1 ASC)
More information on MSDN about creating indexes here
Update thanks to comments by #GarethD who helped me with this, as I wasn't actually aware of it.
As an extra part of the above TSQL statement, it will increase the speed of your queries if you include the names of the other columns that will be used within the index....
CREATE INDEX IX_myTable_Column_1 ON myTable (Column_1 ASC) INCLUDE (ID, Name)
As GarethD points out, using this SQLFiddle as proof, the execution plan is much quicker as it avoids a "RID" (or Row Identifier) lookup.
More information on MSDN about creating indexes with include columns here
Thank you #GarethD
Would this work faster? When I read this question, this was the code that came to mind:
Select top 1 ID, Name
from table
where Column_1 = (Select min(Column_1) from table)
Do you need to create an index for fields of group by fields in an Oracle database?
For example:
select *
from some_table
where field_one is not null and field_two = ?
group by field_three, field_four, field_five
I was testing the indexes I created for the above and the only relevant index for this query is an index created for field_two. Other single-field or composite indexes created on any of the other fields will not be used for the above query. Does this sound correct?
It could be correct, but that would depend on how much data you have. Typically I would create an index for the columns I was using in a GROUP BY, but in your case the optimizer may have decided that after using the field_two index that there wouldn't be enough data returned to justify using the other index for the GROUP BY.
No, this can be incorrect.
If you have a large table, Oracle can prefer deriving the fields from the indexes rather than from the table, even there is no single index that covers all values.
In the latest article in my blog:
NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: Oracle
, there is a query in which Oracle does not use full table scan but rather joins two indexes to get the column values:
SELECT l.id, l.value
FROM t_left l
WHERE NOT EXISTS
(
SELECT value
FROM t_right r
WHERE r.value = l.value
)
The plan is:
SELECT STATEMENT
HASH JOIN ANTI
VIEW , 20090917_anti.index$_join$_001
HASH JOIN
INDEX FAST FULL SCAN, 20090917_anti.PK_LEFT_ID
INDEX FAST FULL SCAN, 20090917_anti.IX_LEFT_VALUE
INDEX FAST FULL SCAN, 20090917_anti.IX_RIGHT_VALUE
As you can see, there is no TABLE SCAN on t_left here.
Instead, Oracle takes the indexes on id and value, joins them on rowid and gets the (id, value) pairs from the join result.
Now, to your query:
SELECT *
FROM some_table
WHERE field_one is not null and field_two = ?
GROUP BY
field_three, field_four, field_five
First, it will not compile, since you are selecting * from a table with a GROUP BY clause.
You need to replace * with expressions based on the grouping columns and aggregates of the non-grouping columns.
You will most probably benefit from the following index:
CREATE INDEX ix_sometable_23451 ON some_table (field_two, field_three, field_four, field_five, field_one)
, since it will contain everything for both filtering on field_two, sorting on field_three, field_four, field_five (useful for GROUP BY) and making sure that field_one is NOT NULL.
Do you need to create an index for fields of group by fields in an Oracle database?
No. You don't need to, in the sense that a query will run irrespective of whether any indexes exist or not. Indexes are provided to improve query performance.
It can, however, help; but I'd hesitate to add an index just to help one query, without thinking about the possible impact of the new index on the database.
...the only relevant index for this query is an index created for field_two. Other single-field or composite indexes created on any of the other fields will not be used for the above query. Does this sound correct?
Not always. Often a GROUP BY will require Oracle to perform a sort (but not always); and you can eliminate the sort operation by providing a suitable index on the column(s) to be sorted.
Whether you actually need to worry about the GROUP BY performance, however, is an important question for you to think about.