I have to do a SELECT on a table like this:
id
username
speed
is_running
The statement is like:
SELECT *
FROM mytable
WHERE username = 'foo'
AND is_running = 1
I have an index on "username". If I'm running the above statement, do I need to also index "is_running" for best performance? Or does only the first column of the select make a difference? I'm using mysql 5.0.
It depends on the type of data you're storing. If it's bool, you may not see a gain from an index on that column alone. You may want to try to add a composite index on the two columns:
ALTER TABLE mytable ADD INDEX `IDX_USERNAME_IS_RUNNING` ( `username` , `is_running` );
It will ultimately depend on the amount of data in the table as to if you require the index. In many cases, the engine might just do a table scan and omit your index all together if it thinks that is faster. Do you have 100 users, or 100,000 users?
On a bit/bool column you are not going to utilize a ton of storage space for the index, so it probably won't hurt unless you have a really high insertion rate.
If you have a table with 1 million users and only 1 or 2 is running at any one time - sure, index by is_running and it will give you fantastic performance. This specific use case would do best to have 2 indexes individually on columns - username, isrunning. The reason for 2 indexes is if you are asking for is_running=0, in which case it uses the username index instead.
Otherwise, there is 0% chance of a composite index on (username, isrunning) helping anything. Stick to a single index on username.
Finally, do you really need to whole record? Select *? In some scenarios close to the tipping point (when MySQL thinks the index+lookups becomes less efficient than a straight scan), you can make this query run faster than the original. Have an index on (username,id)
SELECT mytable.*
FROM (
SELECT id
FROM mytable
WHERE username = 'foo'
AND is_running = 1
) X
INNER JOIN mytable on mytable.id = X.id
Related
My first time working with indexes in database and so far I've learn that if you have a multi-column index such as index('col1', 'col2', 'col3'), and if you do a query that uses where col2='col2' and col3='col3', that index would not be use.
I also learn that if a column is very low selectivity column. Indexing is useless.
However, from my test, it seems none of the above is true at all. Can someone explain more on this?
I have a table with more than 16 million records. Let's say claimID is the primary key, then there're a historynumber column that only have 3 distinct values (1,2,3), and a last column with storeNumber that has about 1 million distinct values.
I have an index for claimID alone, another index(historynumber, claimID), and other index with index(historynumber, storeNumber), and finally index(storeNumber, historynumber).
My guess was that if I do:
select * from my_table where claimId='123456' and historynumber = 1
would be much faster than
select * from my_table where historynumber = 1 and claimId = '123456'
However, the 2 have exactly the same performance (instant). So I thought the primary key index can work on any column order. Therefore, I tried the same thing but on historynumber and storeNumber instead. The result is exactly the same. Then I start trying out on columns that has no indexes and of course the result is the same also.
Finally, I do a
select * from my_table where historynumber = 1
and the query takes so long I had to cancel it.
So my conclusion is that the column order in where clause is completely useless, and so is the column order in the index definition since it seems like the database is smart enough to tell which column is the highest selectivity column.
Could someone give me an example that could prove otherwise?
Index explanation is a huge topic.
Don't worry about the sequence of different attributes in the SQL - it has no effect whether you specify
...where claimId='123456' and historynumber = 1
or the other way round. Each SQL is checked and optimized by the optimizer. To proove how the data gets accessed you could do a EXPLAIN. Check the documentation for more details.
For your other problem
select * from my_table where historynumber = 1
with an index of (storeNumber, historynumber).
Have you ever tried to lookup the name of a caller (having the telephone number) in a telephone book?
Well it is pretty much the same for an index - so the column order when creatin the index matters!
There are techniques which could help - i.e. index jump scan - but there is no guarantee.
Check out following sites to learn a little bit more about DB2 indexes:
http://db2commerce.com/2013/09/19/db2-luw-basics-indexes/
http://use-the-index-luke.com/sql/where-clause/the-equals-operator/concatenated-keys
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've created an Oracle Text index like the following:
create index my_idx on my_table (text) indextype is ctxsys.context;
And I can then do the following:
select * from my_table where contains(text, '%blah%') > 0;
But lets say we have a have another column in this table, say group_id, and I wanted to do the following query instead:
select * from my_table where contains(text, '%blah%') > 0 and group_id = 43;
With the above index, Oracle will have to search for all items that contain 'blah', and then check all of their group_ids.
Ideally, I'd prefer to only search the items with group_id = 43, so I'd want an index like this:
create index my_idx on my_table (group_id, text) indextype is ctxsys.context;
Kind of like a normal index, so a separate text search can be done for each group_id.
Is there a way to do something like this in Oracle (I'm using 10g if that is important)?
Edit (clarification)
Consider a table with one million rows and the following two columns among others, A and B, both numeric. Lets say there are 500 different values of A and 2000 different values of B, and each row is unique.
Now lets consider select ... where A = x and B = y
An index on A and B separately as far as I can tell do an index search on B, which will return 500 different rows, and then do a join/scan on these rows. In any case, at least 500 rows have to be looked at (aside from the database being lucky and finding the required row early.
Whereas an index on (A,B) is much more effective, it finds the one row in one index search.
Putting separate indexes on group_id and the text I feel only leaves the query generator with two options.
(1) Use the group_id index, and scan all the resulting rows for the text.
(2) Use the text index, and scan all the resulting rows for the group_id.
(3) Use both indexes, and do a join.
Whereas I want:
(4) Use the (group_id, "text") index to find the text index under the particular group_id and scan that text index for the particular row/rows I need. No scanning and checking or joining required, much like when using an index on (A,B).
Oracle Text
1 - You can improve performance by creating the CONTEXT index with FILTER BY:
create index my_idx on my_table(text) indextype is ctxsys.context filter by group_id;
In my tests the filter by definitely improved the performance, but it was still slightly faster to just use a btree index on group_id.
2 - CTXCAT indexes use "sub-indexes", and seem to work similar to a multi-column index. This seems to be the option (4) you're looking for:
begin
ctx_ddl.create_index_set('my_table_index_set');
ctx_ddl.add_index('my_table_index_set', 'group_id');
end;
/
create index my_idx2 on my_table(text) indextype is ctxsys.ctxcat
parameters('index set my_table_index_set');
select * from my_table where catsearch(text, 'blah', 'group_id = 43') > 0
This is likely the fastest approach. Using the above query against 120MB of random text similar to your A and B scenario required only 18 consistent gets. But on the downside, creating the CTXCAT index took almost 11 minutes and used 1.8GB of space.
(Note: Oracle Text seems to work correctly here, but I'm not familiar with Text and I can't gaurentee this isn't an inappropriate use of these indexes like #NullUserException said.)
Multi-column indexes vs. index joins
For the situation you describe in your edit, normally there would not be a significant difference between using an index on (A,B) and joining separate indexes on A and B. I built some tests with data similar to what you described and an index join required only 7 consistent gets versus 2 consistent gets for the multi-column index.
The reason for this is because Oracle retrieves data in blocks. A block is usually 8K, and an index block is already sorted, so you can probably fit the 500 to 2000 values in a few blocks. If you're worried about performance, usually the IO to read and write blocks is the only thing that matters. Whether or not Oracle has to join together a few thousand rows is an inconsequential amount of CPU time.
However, this doesn't apply to Oracle Text indexes. You can join a CONTEXT index with a btree index (a "bitmap and"?), but the performance is poor.
I'd put an index on group_id and see if that's good enough. You don't say how many rows we're talking about or what performance you need.
Remember, the order in which the predicates are handled is not necessarily the order in which you wrote them in the query. Don't try to outsmart the optimizer unless you have a real reason to.
Short version: There's no need to do that. The query optimizer is smart enough to decide what's the best way to select your data. Just create a btree index on group_id, ie:
CREATE INDEX my_group_idx ON my_table (group_id);
Long version: I created a script (testperf.sql) that inserts 136 rows of dummy data.
DESC my_table;
Name Null Type
-------- -------- ---------
ID NOT NULL NUMBER(4)
GROUP_ID NUMBER(4)
TEXT CLOB
There is a btree index on group_id. To ensure the index will actually be used, run this as a dba user:
EXEC DBMS_STATS.GATHER_TABLE_STATS('<YOUR USER HERE>', 'MY_TABLE', cascade=>TRUE);
Here's how many rows each group_id has and the corresponding percentage:
GROUP_ID COUNT PCT
---------------------- ---------------------- ----------------------
1 1 1
2 2 1
3 4 3
4 8 6
5 16 12
6 32 24
7 64 47
8 9 7
Note that the query optimizer will use an index only if it thinks it's a good idea - that is, you are retrieving up to a certain percentage of rows. So, if you ask it for a query plan on:
SELECT * FROM my_table WHERE group_id = 1;
SELECT * FROM my_table WHERE group_id = 7;
You will see that for the first query, it will use the index, whereas for the second query, it will perform a full table scan, since there are too many rows for the index to be effective when group_id = 7.
Now, consider a different condition - WHERE group_id = Y AND text LIKE '%blah%' (since I am not very familiar with ctxsys.context).
SELECT * FROM my_table WHERE group_id = 1 AND text LIKE '%ipsum%';
Looking at the query plan, you will see that it will use the index on group_id. Note that the order of your conditions is not important:
SELECT * FROM my_table WHERE text LIKE '%ipsum%' AND group_id = 1;
Generates the same query plan. And if you try to run the same query on group_id = 7, you will see that it goes back to the full table scan:
SELECT * FROM my_table WHERE group_id = 7 AND text LIKE '%ipsum%';
Note that stats are gathered automatically by Oracle every day (it's scheduled to run every night and on weekends), to continually improve the effectiveness of the query optimizer. In short, Oracle does its best to optimize the optimizer, so you don't have to.
I do not have an Oracle instance at hand to test, and have not used the full-text indexing in Oracle, but I have generally had good performance with inline views, which might be an alternative to the sort of index you had in mind. Is the following syntax legit when contains() is involved?
This inline view gets you the PK values of the rows in group 43:
(
select T.pkcol
from T
where group = 43
)
If group has a normal index, and doesn't have low cardinality, fetching this set should be quick. Then you would inner join that set with T again:
select * from T
inner join
(
select T.pkcol
from T
where group = 43
) as MyGroup
on T.pkcol = MyGroup.pkcol
where contains(text, '%blah%') > 0
Hopefully the optimizer would be able to use the PK index to optimize the join and then appy the contains predicate only to the group 43 rows.
If I have an table
create table sv ( id integer, data text )
and an index:
create index myindex_idx on sv (id,text)
would this still be usefull if I did a query
select * from sv where id = 10
My reason for asking is that i'm looking through a set of tables with out any indexes, and seeing different combinations of select queries. Some uses just one column other has more than one. Do I need to have indexes for both sets or is an all-inclusive-index ok?
I am adding the indexes for faster lookups than full table scans.
Example (based on the answer by Matt Huggins):
select * from table where col1 = 10
select * from table where col1 = 10 and col2=12
select * from table where col1 = 10 and col2=12 and col3 = 16
could all be covered by index table (co1l1,col2,col3) but
select * from table where col2=12
would need another index?
It should be useful since an index on (id, text) first indexes by id, then text respectively.
If you query by id, this index will be used.
If you query by id & text, this index will be used.
If you query by text, this index will NOT be used.
Edit: when I say it's "useful", I mean it's useful in terms of query speed/optimization. As Sune Rievers pointed out, it will not mean you will get a unique record given just ID (unless you specify ID as unique in your table definition).
Oracle supports a number of ways of using an index, and you ought to start by understanding all of them so have a quick read here: http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#sthref973
Your query select * from table where col2=12 could usefully leverage an index skip scan if the leading column is of very low cardinality, or a fast full index scan if it is not. These would probably be fine for running reports, however for an OLTP query it is likely that you would do better to create an index with col2 as the leading column.
I assume id is primary key. There is no point in adding a primary key to the index, as this will always be unique. Adding something unique to something else will also be unique.
Add a unique index to text, if you really need it, otherwise just use id is uniqueness for the table.
If id is not your primary key, then you will not be guaranteed to get a unique result from your query.
Regarding your last example with lookup on col2, I think you could need another index. Indexes are not a cure-all solution for performance problems though, sometimes your database design or your queries needs to be optimized, for instance rewritten into stored procedures (while I'm not totally sure Oracle has them, I'm sure there's an Oracle equivalent).
If the driver behind your question is that you have a table with several columns and any combination of these columns may be used in a query, then you should look at BITMAP indexes.
Looking at your example:
select * from mytable where col1 = 10 and col2=12 and col3 = 16
You could create 3 bitmap indexes:
create bitmap index ix_mytable_col1 on mytable(col1);
create bitmap index ix_mytable_col2 on mytable(col2);
create bitmap index ix_mytable_col3 on mytable(col3);
These bitmap indexes have the great benefit that they can be combined as required.
So, each of the following queries would use one or more of the indexes:
select * from mytable where col1 = 10;
select * from mytable where col2 = 10 and col3 = 16;
select * from mytable where col3 = 16;
So, bitmap indexes may be an option for you. However, as David Aldridge pointed out, depending on your particular data set a single index on (col1,col2,col3) might be preferable. As ever, it depends. Take a look at your data, the likely queries against that data, and make sure your statistics are up to date.
Hope this helps.
I have a table with about 20+ million records.
Structure is like:
EventId UNIQUEIDENTIFIER
SourceUserId UNIQUEIDENTIFIER
DestinationUserId UNIQUEIDENTIFIER
CreatedAt DATETIME
TypeId INT
MetaId INT
Table is receiving about 100k+ records each day.
I have indexes on each column except MetaId, as it is not used in 'where' clauses
The problem is when i want to pick up eg. latest 100 records for desired SourceUserId
Query sometimes takes up to 4 minutes to execute, which is not acceptable.
Eg.
SELECT TOP 100 * FROM Events WITH (NOLOCK)
WHERE SourceUserId = '15b534b17-5a5a-415a-9fc0-7565199c3461'
AND
(
TypeId IN (2, 3, 4)
OR
(TypeId = 60 AND SrcMemberId != DstMemberId)
)
ORDER BY CreatedAt DESC
I can't do partitioning etc as I am using Standard version of SQL Server and Enterprise is too expensive.
I also think that the table is quite small to be that slow.
I think the problem is with ORDER BY clause as db must go through much bigger set of data.
Any ideas how to make it quicker ?
Perhaps relational database is not a good idea for that kind of data.
Data is always being picked up ordered by CreatedAt DESC
Thank you for reading.
PabloX
You'll likely want to create a composite index for this type of query - when the query runs slowly it is most likely choosing to scan down an index on the CreatedAt column and perform a residual filter on the SourceUserId value, when in reality what you want to happen is to jump directly to all records for a given SourceUserId ordered properly - to achieve this, you'll want to create a composite index primarily on SourceUserId (performing an equality check) and secondarily on CreateAt (to preserve the order within a given SourceUserId value). You may want to try adding the TypeId in as well, depending on the selectivity of this column.
So, the 2 that will most likely give the best repeatable performance (try them out and compare) would be:
Index on (SourceUserId, CreatedAt)
Index on (SourceUserId, TypeId, CreatedAt)
As always, there are also many other considerations to take into account with determining how/what/where to index, as Remus discusses in a separate answer one big consideration is covering the query vs. keeping lookups. Additionally you'll need to consider write volumes, possible fragmentation impact (if any), singleton lookups vs. large sequential scans, etc., etc.
I have indexes on each column except
MetaId
Non-covering indexes will likely hit the 'tipping point' and the query would revert to a table scan. Just adding an index on every column because it is used in a where clause does not equate good index design. To take your query for example, a good 100% covering index would be:
INDEX ON (SourceUserId , CreatedAt) INCLUDE (TypeId, SrcMemberId, DstMemberId)
Following index is also usefull, altough it still going to cause lookups:
INDEX ON (SourceUserId , CreatedAt) INCLUDE (TypeId)
and finaly an index w/o any included column may help, but is just as likely will be ignored (depends on the column statistics and cardinality estimates):
INDEX ON (SourceUserId , CreatedAt)
But a separate index on SourceUSerId and one on CreatedAt is basically useless for your query.
See Index Design Basics.
The fact that the table has indexes built on GUID values, indicates a possible series of problems that would affect performance:
High index fragmentation: since new GUIDs are generated randomly, the index cannot organize them in a sequential order and the nodes are spread unevenly.
High number of page splits: the size of a GUID (16 bytes) causes many page splits in the index, since there's a greater chance than a new value wont't fit in the remaining space available in a page.
Slow value comparison: comparing two GUIDs is a relatively slow operation because all 33 characters must be matched.
Here a couple of resources on how to investigate and resolve these problems:
How to Detect Index Fragmentation in SQL Server 2000 and 2005
Reorganizing and Rebuilding Indexes
How Using GUIDs in SQL Server Affect Index Performance
I would recomend getting the data in 2 sep var tables
INSERT INTO #Table1
SELECT * FROM Events WITH (NOLOCK)
WHERE SourceUserId = '15b534b17-5a5a-415a-9fc0-7565199c3461'
AND
(
TypeId IN (2, 3, 4)
)
INSERT INTO #Table2
SELECT * FROM Events WITH (NOLOCK)
WHERE SourceUserId = '15b534b17-5a5a-415a-9fc0-7565199c3461'
AND
(
(TypeId = 60 AND SrcMemberId != DstMemberId)
)
then apply a unoin from the selects, ordered and top. Limit the data from the get go.
I suggest using a UNION:
SELECT TOP 100 x.*
FROM (SELECT a.*
FROM EVENTS a
WHERE a.typeid IN (2, 3, 4)
UNION ALL
SELECT b.*
FROM EVENTS b
WHERE b.typeid = 60
AND b.srcmemberid != b.dstmemberid) x
WHERE x.sourceuserid = '15b534b17-5a5a-415a-9fc0-7565199c3461'
We've realised a minor gain by moving to a BIGINT IDENTITY key for our event table; by using that as a clustered primary key, we can cheat and use that for date ordering.
I would make sure CreatedAt is indexed properly
you could split the query in two with an UNION to avoid the OR (which can cause your index not to be used), something like
SElect * FROM(
SELECT TOP 100 * FROM Events WITH (NOLOCK)
WHERE SourceUserId = '15b534b17-5a5a-415a-9fc0-7565199c3461'
AND TypeId IN (2, 3, 4)
UNION SELECT TOP 100 * FROM Events WITH (NOLOCK)
WHERE SourceUserId = '15b534b17-5a5a-415a-9fc0-7565199c3461'
AND TypeId = 60 AND SrcMemberId != DstMemberId
)
ORDER BY CreatedAt DESC
Also, check that the uniqueidentifier indexes are not CLUSTERED.
If there are 100K records added each day, you should check your index fragmentation.
And rebuild or reorganize it accordingly.
More info :
SQLauthority