What happens if I put index on each column in a table - mysql

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.

Related

Should I setup index on a table having 15k -30k row?

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

Order of columns in an index

I am working on a database with large number of rows (6 Mil+).
This table has a composite primary key on two columns.
It also has separate index on each of those fields as there are queries that require this. Obviously, one of those indexes (indices?) is redundant and slowing down performance for write operations.
How do I find out which one is redundant? I understand the first column of a primary key is already indexed and need not be indexed separately. Is that correct? If so, is there a query I can run to find out which is the first one in the list?
SHOW INDEXES FROM tablename will include a Seq_in_index column, which tells you which is first (aka, left most) column, second column, etc.
Therefore, whichever column is listed with a value of 1 for Seq_in_index is the column that does not need it's own single column index.
You can also use SHOW CREATE TABLE tablename to see the index listed from left to right, and that order displayed correctly represents the order of columns in the index.
SHOW CREATE TABLE tablename gives you all the indexes, in their established order.
You don't need INDEX(a) because the column(s) in it are the first column(s) in the INDEX(a,b),
That applies to INDEX / UNIQUE / PRIMARY KEY in (a,b).
I understand the first column of a primary key is already indexed
Erm, no. All the columns in the primary key are indexed.
An explanation of how indexes work is stretching the scope of a post here, and the question of which indexes to put on your table is way too broad.
Suppose you have a primary key defined on attributes a,b,c. This index can be used for queries with predicates
a
a and b
a and b and c
But (at least, the last time I checked) it would not be used for a query with predicates
b
b and c
The optimizer will only ever use one index for each table in a query.
The right indexes depend on the volume of data, the cardinality of the data and the frequency and combination of predicates in your queries. There are execution and storage overheads when you start adding indexes, even just for select operations badly designed indexes can make your query slower than it would run without indexes.

use Foreign key indexes in multi index mysql

I have A table with almost 20 fields which several of those are Foreign Key that already has been indexed by Mysql, now I want to create a multi-indexes index that it contains 3 FK field,
First tried was based on Fields
ALTER TABLE `Add`
Add INDEX `IX_Add_ON_IDCat_IDStatus_IDModeration_DateTo_DateAdded`
(`IDCategory`,`IDStatus`,`IDModeration`,`DateTo`,`DateAdded`);
But I think it's better to have an index on indexes instead of fields but my following effort faced with error: Error Code: 1072. Key column 'FK_Add_Category' doesn't exist in table
ALTER TABLE `Add`
Add INDEX `IX_Add_ON_IDCat_IDStatus_IDModeration_DateTo_DateAdded`
(`FK_Add_Category`,`FK_Add_AddStatus`,`FK_Add_AddModeration`,
`IX_Add_DateTo`,`IX_Add_DateAdded`);
My question is is it possible to add an index on exists Indexes ( FK index in my case ) or not and there is the only way to create an index on Columns? if yes How I create that?
An index is an ordered list of values. It is used to make it more efficient to find rows in the table.
Think about the common, real-life, example of INDEX(last_name, first_name). It makes it easy to look up someone if you have their last name and first name. And sort of easy if you have only their last name.
But it is useless if all you have is their first name.
FOREIGN KEYs necessitate a lookup. Apparently you have a FK to AddStatus, since I see FK_Add_AddStatus. That FK generated a lookup for AddStatus. Think of that as being like a separate index on first_name. It is totally separate from the index on last_name & first_name.
5 columns is usually too many to put into a single index.
MySQL uses only one index for a given SELECT.
So, now, I ask, what SELECT might use that 5-column index? Please show us it. We can discuss whether it is useful, and whether the columns are in the optimal order.

Use an index on a join without "where"

I'm trying to understand if it's possible to use an index on a join if there is no limiting where on the first table.
Note: this is not a line-by-line real-case usage, just a thing I draft together for understanding purposes. Don't point out the obvious "what are your trying to obtain with this schema?", "you should use UNSIGNED" or the likes because that's not the question.
Note2: this MySQL JOINS without where clause is somehow related but not the same
Schema:
CREATE TABLE posts (
id_post INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
text VARCHAR(100)
);
CREATE TABLE related (
id_relation INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
id_post1 INT NOT NULL,
id_post2 INT NOT NULL
);
CREATE INDEX related_join_index ON related(id_post1) using BTREE;
Query:
EXPLAIN SELECT * FROM posts FORCE INDEX FOR JOIN(PRIMARY) INNER JOIN related ON id_post=id_post1 LIMIT 0,10;
SQL Fiddle: http://sqlfiddle.com/#!2/84597/3
As you can see, the index is being used on the second table, but the engine is doing a full table scan on the first one (the FORCE INDEX is there just to highlight the general question).
I'd like to understand if it's possible to get a "ref" on the left side too.
Thanks!
Update: if the first table has significantly more record than the second, the thing swap: the engine uses an index for the first one and a full table scan for the second http://sqlfiddle.com/#!2/3a3bb/1 Still, no way to get indexes used on both.
The DBMS has an optimizer to figure out the best plan to execute a query. It's up to the optimizer to decide whether to use an index or simply read the table directly.
An index makes sense when the DBMS expects only few records to read from a table (say 1% of all rows only). But once it expects to read many records (say 99% of all rows) it will not use the index. The threshold may lie at low as 5% (i.e. <= 5% -> index; > 5% table scan).
There are exceptions. One is when an index holds all columns needed. Then the table itself doesn't have to be read at all. Another may be when the optimizer thinks an index access may result faster in spite of having to read many rows. It's also always possible the optimizer simply guesses wrong.
There is a page on the MySQL documentation about this subject.
Regarding the possibility to get a ref on the first table from the query, the short answer is NO.
The reason is obvious: because there is no WHERE clause ALL the rows from table posts are analyzed because they could be included in the result set. There is no reason to use an index for that, a full table scan is better because it gets all the rows; and because the order doesn't matter, the access is (more or less) sequential. Using an index requires reading more information from the storage (index and data).
MySQL will use the join type index if all the columns that appear in the SELECT clause are present in an index. In this case MySQL will perform a full index scan (join type index) instead of a full table scan (join type ALL) because it requires reading less information from the storage (an index is usually smaller than the entire table data).

phpMyAdmin wants to add multiple indices as one multi-column index

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.