Is it possible to create two indexes with names different on the same column?
Yes, you can, but why would you do that?
Unless the indexes are different in some way, for example if there are additional columns, or differences in the order of the columns in the indexes, a second duplicated index would be redundant.
Each additional index on a table requires more disk storage (slight cost increase), and also means more data needs to be written when inserting, updating or deleting data (slightly slower writes).
But yes, it is possible, and the syntax is one would expect, e.g. given the table:
CREATE TABLE T1
(
col1 INT,
col2 INT
);
CREATE INDEX IX1 on T1(col1);
CREATE INDEX IX2 on T1(col1);
SQL Fiddle here
Related
I have a table we will be of 15-30k in size eventually not more.
I have only two columns in that table one is id and other is status
We will have insert queries ofc
We will have update query but not on id
We will have delete query
My question is should I create index on the column id ?
Will index be useful for a table having 15k-30k rows ? Or will it be negligible ?
Also I am concerned about the increase in cost of insert queries? Will it be worth to add index on id ? Considering the small table size will it be any faster or the effect will be negligible ?
If effect will be negligible, I should not add index to it right as it will increase the insert queries cost right ?
If your id column is a PRIMARY KEY, then it's already an index and there is no need to create a new one.
If no primary key is defined, it's best to get used of creating one for each table.
Without any index, MySQL has to start with the first row and go through the whole table to find the relevant rows.
Indexes make it possible to find data much faster, even on tables with few data.
MySQL 8.0 Reference Manual:
https://dev.mysql.com/doc/refman/8.0/en/mysql-indexes.html
I have a table with index on a int column.
Create table sample(
col1 varchar,
col2 int)
Create index idx1 on sample(col2);
When I explain the following query
Select * from sample where col2>2;
It does a full table scan.
Why doesn't the indexing work here?
How can i optimize such queries when table has around 20 million records?
Just because you create an index, does not mean MySQL will always use it. According to the docs, here are several reasons why it may choose to use a full table scan over the index:
The table is so small that it is faster to perform a table scan than to bother with a key lookup. This is common for tables with fewer than 10 rows and a short row length.
There are no usable restrictions in the ON or WHERE clause for indexed columns.
You are comparing indexed columns with constant values and MySQL has calculated (based on the index tree) that the constants cover too large a part of the table and that a table scan would be faster. See Section 8.2.1.1, “WHERE Clause Optimization”.
You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key it probably will do many key lookups and that a table scan would be faster.
You can use FORCE INDEX to ensure your query uses the index instead of allowing the optimizer to determine the appropriate path, although usually MySQL will take the most efficient approach.
SELECT * FROM t1, t2 FORCE INDEX (index_for_column) WHERE t1.col_name=t2.col_name;
Reference: https://dev.mysql.com/doc/refman/8.0/en/table-scan-avoidance.html
I’m having trouble understanding exactly what happens when I create an index. So if I create an index on (Col1, Col2). Will there be a new table created containing only Col1 and Col2 which my query will run on?
Or do I specify that I’m using an index in my query and then for that query and that query only my table is now reduced to (Col1, Col2)?
Thanks
When you create an index on a table, internally it doesn't create any new table but just index. This index is physically separate and has data organized like a tree (usually B+ tree) which helps in faster lookups and speeds up the queries that have where clause having the column name on which index are created. If you don't create indexes on table then the queries on that table may have to do full table scan in order to find records. Indexes can be unique or non unique.
I am creating a table which will store around 100million rows in MySQL 5.6 using InnoDB storage engine. This table will have a foreign key that will link to another table with around 5 million rows.
Current Table Structure:
`pid`: [Foreign key from another table]
`price`: [decimal(9,2)]
`date`: [date field]
and every pid should have only one record for a date
What is the best way to create indexes on this table?
Option #1: Create Primary index on two fields pid and date
Option #2: Add another column id with AUTO_INCREMENT and primary index and create a unique index on column pid and date
Or any other option?
Only select query i will be using on this table is:
SELECT pid,price,date FROM table WHERE pid = 123
Based on what you said (100M; the only query is...; InnoDB; etc):
PRIMARY KEY(pid, date);
and no other indexes
Some notes:
Since it is InnoDB, all the rest of the fields are "clustered" with the PK, so a lookup by pid is acts as if price were part of the PK. Also WHERE pid=123 ORDER BY date would be very efficient.
No need for INDEX(pid, date, price)
Adding an AUTO_INCREMENT gains nothing (except a hint of ordering). If you needed ordering, then an index starting with date might be best.
Extra indexes slow down inserts. Especially UNIQUE ones.
Either method is fine. I prefer having synthetic primary keys (that is, the auto-incremented version with the additional unique index). I find that this is useful for several reasons:
You can have a foreign key relationship to the table.
You have an indicator of the order of insertion.
You can change requirements, so if some pids allows two values per day or only one per week, then the table can support them.
That said, there is additional overhead for such a column. This overhead adds space and a small amount of time when you are accessing the data. You have a pretty large table, so you might want to avoid this additional effort.
I would try with an index that attempts to cover the query, in the hope that MySQL has to access to the index only in order to get the result set.
ALTER TABLE `table` ADD INDEX `pid_date_price` (`pid` , `date`, `price`);
or
ALTER TABLE `table` ADD INDEX `pid_price_date` (`pid` , `price`, `date`);
Choose the first one if you think you may need to select applying conditions over pid and date in the future, or the second one if you think the conditions will be most probable over pid and price.
This way, the index has all the data the query needs (pid, price and date) and its indexing on the right column (pid)
By the way, always use EXPLAIN to see if the query planner will really use the whole index (take a look at the key and keylen outputs)
Let us consider I have a table with 60 columns , I need to perform all kind of queries on that table and need to join that table with other tables as well. And I almost using all rows for searching data in that table including other tables. This table is the like a primary table(like a primary key) in the database. So all table are in relation with this table.
By considering the above scenario can I create index on each column on the table (60 columns )
,is it good practice ?
In single sentence:
Is it best practice to create index on each column in a table ?
What might happens if I create index on each column in a table?
Where index might be "Primary key", "unique key" or "index"
Please comment, if this question is unclear for you people I will try to improve this question.
MySQL's documentation is pretty clear on this (in summary use indices on columns you will use in WHERE, JOIN, and aggregation functions).
Therefore there is nothing inherently wrong with creating an index on all columns in a table, even if it is 60 columns. The more indices there are the slower inserts and some updates will be because MySQL has to create the keys, but if you don't create the indices MySQL has to scan the entire table if only non-indexed columns are used in comparisons and joins.
I have to say that I'm astonished that you would
Have a table with 60 columns
Have all of those columns used either in a JOIN or WHERE clause without dependency on any other column in the same table
...but that's a separate issue.
It is not best practice to create index on each column in a table.
Indexes are most commonly used to improve query performance when the column is used in a where clause.
Suppose you use this query a lot:
select * from tablewith60cols where col10 = 'xx';
then it would be useful to have an index on col10.
Note that primary keys by default have an index on them, so when you join the table with other tables you should use the primary key to join.
Adding an index means that the database has to maintain it, that means that it has to be updated, so the more writes you have, the more the index will be updated.
Creating index out of the box is not a good idea, create an index only when you need it (or when you can see the need in the future... only if it is pretty obvious)
creating more index in SQL will increase only search speed while you will get slowness of insert and update and also it will take more storage.