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.
Related
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
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.
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
I have generated a drop indexes script that pulls from information_schema.statistics to drop the indexes dynamically. It works properly.
However I have been unable to think of a way to recreate the indexes (based on a temp table created by drop indexes script).
For example:
Table Food exists with 3 columns, id is indexed, and there exists a multi-column index of name and category.
My drop script drops the indices [after storing them in a temp table food_temp_indexes]. I then would like to re-generate the indexes, including the multi-column index as well as the id index dynamically. Whether there is one index or 5, varying from a simple index to a primary key to a multi-column, I wish to dynamically re-add these indexes.
The idea is to drop indexes before inserting millions of records, insert them, then recreate the indexes.
I'm creating tables using phpMyAdmin and want to define two different columns as indices. I'm not trying to create a multi-column index but phpMyAdmin creates them as such. Are there any possible issues with that? The fields don't relate to each other directly and both fields will not be used in WHERE clauses simultaneously.
Consider:
ALTER TABLE `documents` ADD INDEX (`offer_number`, `contract_number`);
And:
ALTER TABLE `documents` ADD INDEX (`offer_number`);
ALTER TABLE `documents` ADD INDEX (`contract_number`);
What's the difference?
MySQL can only make use of an index if the first column(s) of the index match the columns used in the query. In other words, if you perform a query where an index on contract_number could be used, the composite index won't be used since contract_number is not the first column in that key. The composite index could be used for a query where offer_number is used, however.
http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html
Given what you say about these fields, they should not be a part of one multi column index.
If you want to create single column indexes on PhpMyAdmin, you need to create them one at a time.