this is myTable
clientId, itemId, sellId
primary is on clientId and there is also btree index no uniqe on sellId
now i have part of very slow query
LEFT OUTER JOIN myTable wl ON wl.itemId= ld.itemId and wl.clientId= #clientId
question is should i create here index combined for both clientId and itemId or since clientId is primary then only for itemId ?
Your question is this: should I put the primary key PK of the table into a multi-column index as the last column?
If you use the InnoDB storage engine, the answer is no. Why not? InnoDB already includes the PK as part of the index.
If you don't use InnoDB -- that is, if you use MyISAM or AriaDB (in MariaDB), the answer is yes.
Still, you should evaluate how well the index helps your query.
ON wl.itemId= ld.itemId and wl.clientId= #clientId
begs for
INDEX(itemId, clientId) -- in either order
However, if either of those columns is the PRIMARY KEY of wl, then no index is needed, nor useful. The PK will provide immediate access to the needed row. The other column cannot do anything other than verify that it matches -- that is eliminate the row from the JOIN.
should i create here index combined for both clientId and itemId
Yes, if neither is UNIQUE (Keep in mind that the PK is 'unique'.)
or since clientId is primary then only for itemId
Almost never will MySQL use two separate indexes. (Keep in mind that the PK is an index.)
Related
When I join 2 tables tbl1, tbl2 on column1, where column1 is primary key on tbl1. Assuming that column1 is not automatically indexed should I create an index on both tbl1.column1 and tbl2.column1 or just on tbl2.column1. Are the number of rows of each table affect that choice?
A primary key is automatically indexed. There is no way around that (this is how the "unique" part of the unique constraint is implemented). So, tbl1.column1 has an index. No other index is needed.
As for tbl2.column2, you should probably have an index on that. MySQL does create an index if you explicitly declare a foreign key relationship. So, with the explicit declaration, no other index is necessary. Note: this is not true of all databases.
The presence of indexes does not change the results of queries nor the number of rows in the table, so I don't understand your final question. Indexes implement relational integrity and improve (hopefully!) performance on certain types of queries
Generally yes, because often you'll want to do the reverse of the join at some point.
In mysql can I have a composite primary key composed of an auto increment and another field? Also, please critique my “mysql partitioning” logic
To explain further->
I have a query about MySQL partition.
I have to partition a table in MySQL, It has one primary key id.
I have to partition by date field(non-primary,duplicate entries).
Since we cannot partition on duplicate entries, i have created a composite key->(id,date).
How can i create partition in this composite key?
Thanks in Advance...
(This answer assumes InnoDB, not MyISAM. There are differences in the implementation of indexes that make some of my comments incorrect for MyISAM.)
In MySQL, a table's PRIMARY KEY can be composed of multiple fields, including an AUTO_INCREMENT.
The only requirement in MySQL for AUTO_INCREMENT is that it be the first column in some index. Let's look at this example of Posts, where there can be many posts for each user:
PRIMARY KEY(user_id, post_id),
INDEX(post_id)
where post_id is AUTO_INCREMENT, but you could benefit from "clustering" the data by user_id. This clustering would make it more efficient to do queries like
SELECT ... FROM Posts
WHERE user_id = 1234;
Back to your question...
The "partition key" does not have to be unique; so, I don't understant your "cannot partition on duplicate entries".
INDEX(id, date), if you also have PRIMARY KEY(id), is essentially useless. When looking up by id, the PRIMARY KEY(id) gives you perfect access; adding date to an index won't help. When looking up by date, but not id, (id, date) is useless since only the "left" part of a composite index can be used.
Perhaps you are leading to a non-partitioned table with
PRIMARY KEY(date, id),
INDEX(id)
to make date ranges efficient? (Note: partitioning won't help.)
Perhaps you will be doing
SELECT ... WHERE x = 123 AND date BETWEEN ...
In that case this is beneficial:
INDEX(x, date)
Only if you do this can we begin to discuss the utility of partitioning:
WHERE x BETWEEN ...
AND date BETWEEN ...
This needs a "two-dimensional" index, which sort of exists with SPATIAL.
See my discussion of partitioning where I list only 4 use cases for partitioning. It also links to an a discussion on how to use partitioning for 2D.
Bottom Line: You must not discuss partitioning without having a clear picture of what queries it might help. Provide them; then we can discuss further.
So I wonder should I add non-clustered index to a non-unique values column in SQL 2008 R2 table.
Simplified Example:
SELECT Id, FirstName, LastName, City
FROM Customers
WHERE City = 'MyCity'
My understanding is that the primary key [Id] should be the clustered index.
Can non-clustered index be added to the non-unique column [City] ?
Is this going to improve performance or I should not bother at all.
Thanks.
I was thinking to do clustered index as:
CREATE UNIQUE CLUSTERED INDEX IDX_Customers_City
ON Customers (City, Id);
or non-clustered, assuming there is already clustered index on that table.
CREATE NONCLUSTERED INDEX IX_Customers_City
ON Customers (City, Id);
In reality I am dealing with millions of records table. The Select statement returns 0.1% to 5% of the records
Generally yes - you would usually make the clustered index on the primary key.
The exception to this is when you never make lookups based on the primary key, in which case putting the clustered index on another column might be more pertinent.
You should generally add non-clustered indexes to columns that are used as foreign keys, providing there's a reasonably amount of diversity on that column, which I'll explain with an example.
The same applies to columns being used in where clauses, order by etc.
Example
CREATE TABLE Gender (
GenderId INT NOT NULL PRIMARY KEY CLUSTERED
Value NVARCHAR(50) NOT NULL)
INSERT Gender(Id, Value) VALUES (1, 'Male'), (2, 'Female')
CREATE TABLE Person (
PersonId INT NOT NULL IDENTITY(1,1) PRIMARY KEY CLUSTERED,
Name NVARCHAR(50) NOT NULL,
GenderId INT NOT NULL FOREIGN KEY REFERENCES Gender(GenderId)
)
CREATE TABLE Order (
OrderId INT NOT NULL IDENTITY(1,1) PRIMARY KEY CLUSTERED,
OrderDate DATETIME NOT NULL DEFAULT GETDATE(),
OrderTotal DECIMAL(14,2) NOT NULL,
OrderedByPersonId INT NOT NULL FOREIGN KEY REFERENCES Person(PersonId)
)
In this simple set of tables it would be a good idea to put an index on The OrderedByPersonId Column of the Order table, as you are very likely to want to retrieve all the orders for a given person, and it is likely to have a high amount of diversity.
By a high amount of diversity (or selectiveness) I mean that if you have say 1000 customers, each customer is only likely to have 1 or 2 orders each, so looking up all the values from the order table with a given OrderedByPersonId will result in only a very small proportion of the total records in that table being returned.
By contrast there's not much point in putting an index on the GenderId column in the Person table, as it will have a very low diversity. The query optimiser would not use such an index, and INSERT/UPDATE statements would be a marginally slower because of the extra need to maintain the index.
Now to go back to your example - the answer would have to be "it depends". If you have hundreds of cities in your database then yes, it might be a good idea to index that column
If however you only have 3 or 4 cities, then no - don't bother. As a guideline I might say if the selectivity of the column is 0.9 or higher (ie a where clause selecting a single value in the column would result in only 10% or less of the rows being returned) an index might help, but this is by no means a hard and fast figure!
Even if the column is very selective/diverse you might not bother indexing it if queries are only made very infrequently on it.
One of the easiest things to do though is try your queries with the execution plan displayed in SQL management studio. It will suggest indexes for you if the query optimiser thinks that they'll make a positive impact.
Hope that helps!
If you use the query frequently or if you sort by city regularly in on-line applications specially if your table is dense or has a large row size, it makes sense to add an index. Too many indexes slow down your insert and update. An evaluation of the actual value would only be appreciated when you have significant data in the table.
I was wondering when having a parent table and a child table with foreign key like:
users
id | username | password |
users_blog
id | id_user | blog_title
is it ok to use id as auto increment also on join table (users_blog) or will i have problems of query speed?
also i would like to know which fields to add as PRIMARY and which as INDEX in users_blog table?
hope question is clear, sorry for my bad english :P
I don't think you actually need the id column in the users_blog table. I would make the id_user the primary index on that table unless you have another reason for doing so (perhaps the users_blog table actually has more columns and you are just not showing it to us?).
As far as performance, having the id column in the users_blog table shouldn't affect performance by itself but your queries will never use this index since it's very unlikely that you'll ever select data based on that column. Having the id_user column as the primary index will actually be of benefit for you and will speed up your joins and selects.
What's the cardinality between the user and user_blog? If it's 1:1, why do you need an id field in the user_blog table?
is it ok to use id as auto increment also on join table (users_blog)
or will i have problems of query speed?
Whether a field is auto-increment or not has no impact on how quickly you can retrieve data that is already in the database.
also i would like to know which fields to add as PRIMARY and which as
INDEX in users_blog table?
The purpose of PRIMARY KEY (and other constraints) is to enforce the correctness of data. Indexes are "just" for performance.
So what fields will be in PRIMARY KEY depends on what you wish to express with your data model:
If a users_blog row is identified with the id alone (i.e. there is a "non-identifying" relationship between these two tables), put id alone in the PRIMARY KEY.
If it is identified by a combination of id_user and id (aka. "identifying" relationship) then you'll have these two fields together in your PK.
As of indexes, that depends on how you are going to access your data. For example, if you do many JOINs you may consider an index on id_user.
A good tutorial on index performance can be found at:
http://use-the-index-luke.com
I don't see any problem with having an auto increment id column on users_blog.
The primary key can be id_user, id. As for indexing, this heavily depends on your usage.
I doubt you will be having any database related performance issue with a blog engine though, so indexing or not doesn't make much of a difference.
You dont have to use id column in users_blog table you can join the id_user with users table. also auto increment is not a problem to performance
It is a good idea to have an identifier column that is auto increment - this guarantees a way of uniquely identifying the row (in case all other columns are the same for two rows)
id is a good name for all table keys and it's the standard
<table>_id is the standard name for foreign keys - in your case use user_id (not id_user as you have)
mysql automatically creates indexes for columns defined as primary or foreign keys - there is no need to do anything here
IMHO, table names should be singular - ie user not users
You SQL should look something like:
create table user (
id int not null auto_increment primary key,
...
);
create table user_blog (
id int not null auto_increment primary key,
id_user int not null references user,
...
);
I Have 2 Question
If my table contain a unique column like this:
DROP TABLE IF EXISTS TestTable;
CREATE TABLE TestTable(
ID INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
GUID VARCHAR(32) UNIQUE NULL);
Do i Need to create an Index for this GUID column.
Note: i have GUID column In Where statement with join tables
My Second Question is the update statement will effect the index table if the updated column(s) not have been indexes?
No, UNIQUE is kind of index, so you don't need another index on the same column.
It won't update the index, if the changed column is not indexed.
Indexes that are not changed do not get updated.
source
It depends on which database you are using. Different databases have different ways of indexing.
If you are using InnoDB then the Primary Key and Unique Key is already an index, so you won't need to. If you create manually yet another index for the GUID column then you will have an extra redundant index on that column which wastes space.