MySQL multiple index types? - mysql

I've noticed that in PHPMyAdmin I can individually index columns or I can use checkboxes to select fields and then click index and they're indexed in a different way. Does this mean that if for a given table I have 2 columns of that table that define each row as unique (instead of just a simple single column id`) I should index those together to increase performance?

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.
MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you do not specify values for the other columns.
If you have two columns named last_name and first_name and you create an index INDEX name (last_name,first_name), The index can be used for queries that specify values in a known range for last_name, or for both last_name and first_name.
Source: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html
So, it may not be helpful in your particular case. Becuase if you want to query on the later columns (for example: SELECT * FROM test WHERE first_name='Michael' or SELECT * FROM test WHERE last_name='Widenius' OR first_name='Michael), the index will not be used and the queries will be slower.

Related

Displaying entries of an index (MYSQL)

I am trying to list all the entries of an index.
Let's say I have unique index_idx on 2 columns (col_1, col_2) on table table_test.
I assume that the uniqueness for a tuple is checked by concatenating the values in column col1 and col2 and checking if overall the value is unique.
But is there any method predefined using which we can list all the entries for index index_idx.
I saw a similar question here. But the answer didn't make much sense to me. What exactly is ''INDEX COLUMNS LIST' here?
There's no way directly to list the contents of an index (the MySQL / MariaDB developers may have a way to dump index contents, but if they do it's for debugging purposes).
SELECT col_1, col_2 FROM table_test gets you the contents of the index. The index you mentioned is a so-called covering index for that query. MySQL uses the index, rather than the table, to satisfy the query. That's a little faster.
The item you linked does this same thing.
And you are basically correct that the uniqueness of a two-column UNIQUE index is determined with the concatenation of the column values.

How MySQL handles IN predicates

Suppose the table T has three columns,
id int not null auto_increment,
my_id int,
name varchar(200).
and the query is "select * from T where my_id in (var_1, var_2, ..., var_n) and name = 'name_var'".
Is there any performance difference between below two indices?
Index1: (my_id, name)
Index2: (name, my_id).
•Index1: (my_id, name)
•Index2: (name, my_id).
Yes, above two would slightly differ when it comes to query performance.
Always, the leftmost fields are the most important in determining the efficiency and selectivity of an index.
index should be built on the column(s) which are frequently used in the WHERE, ORDER BY, and GROUP BY clauses.
Hope this helps!
In a composite index, the column to be searched should appear first. So, if you are searching for a set of id values, you'll want id to show up first in the index.
But if id is the primary key and you're using a SELECT * clause to retrieve the whole row, it doesn't make sense to add another index. The way tables are organized, all the data of the row appears clustered with each id value. So just use the index on the primary key.
tl;dr: neither (id,name) nor (name,id) will help this query.
In general, it is best to start the INDEX with the WHERE clauses with col = const. One range can come last. IN is sort of like =, sort like a range. Hence, this is best:
INDEX(name, id)
Think of it this way. The index is an ordered list. With this index, it will start at the name=... and then have to scan or leapfrog through all the ids with that name.
I suspect the PRIMARY KEY is not (id). If it were, why would you be checking the name?

MySQL - Does SELECT * need an index of all table fields?

I would like to know if it is necessary to create an index for all fields within a table if one of your queries will use SELECT *.
To explain, if we had a table that 10M records and we did a SELECT * query on it would the query run faster if we have created an index for all fields within the table or does MySQL handle SELECT * in a different way to SELECT first_field, a_field, last_field.
To my understanding, if I had a query that did SELECT first_field, a_field FROM table then it would bring performance benefits if we created an index on first_field, a_field but if we use SELECT * is there even a benefit from creating an index for all fields?
Performing a SELECT * FROM mytable query would have to read all the data from the table. This could, theoretically, be done from an index if you have an index on all the columns, but it would be just faster for the database to read the table itself.
If you have a where clause, having an index on (some of) the columns you have conditions on may dramatically improve the query's performance. It's a gross simplification, but what basically happens is the following:
The appropriate rows are filtered according to the where clause. It's much faster to search for these rows in an index (which is, essentially, a sorted tree) than a table (which is an unordered set of rows).
For the columns that where in the index used in the previous step the values are returned.
For the columns that aren't, the table is accessed (according to a pointer kept in the index).
indexing a mysql table for a column improves performance when there is a need to search or edit a row/record based on that column of that table.
for example, if there is an 'id' column and if it is a primary key; And in that case if you want to search a record using where clause on that 'id' column then you don't need to create index for the 'id' column because primary key column will act as an indexed column.
In another case, if there is an 'pid' column in the table and if it is not a primary key; Then in order to search based on 'pid' column then to improve performance it is better to create an index for the 'pid' column. That will make query fast to search the expected record.

MySql - Is there any benefit to indexing a column that is naturally unique?

Let's use lastName as an example.
Assuming that there are no duplicate last names in your DB (by chance, not because of a unique), would there be any benefit to indexing this lastName column?
The query that would be used to search would be something like SELECT * IN t WHERE lastName='Smith'.
If every entry in the column is unique, then how can an index have an effect? Wouldn't it have to search every entry regardless?
Sorry, I am just learning about indexing and I would really like to understand it better.
Thanks.
Yes, there is a benefit in indexing even if the column values are unique. In the index the values are not only unique but they are also organised in a tree structure that lets you search for a row with O(log N) complexity.
There is a great article in Wikipedia about it: Database Index
...
The data is present in arbitrary order, but the logical ordering is specified
by the index. The data rows may be spread throughout the table
regardless of the value of the indexed column or expression. The
non-clustered index tree contains the index keys in sorted order, with
the leaf level of the index containing the pointer to the record (page
and the row number in the data page in page-organized engines; row
offset in file-organized engines).
In a non-clustered index
The physical order of the rows is not the same as the index order. The
indexed columns are typically non-primary key columns used in JOIN,
WHERE, and ORDER BY clauses. There can be more than one non-clustered
index on a database table.
...
Consider the following SQL statement:
SELECT first_name FROM people WHERE last_name = 'Smith';
To process this statement without an index
the database software must look at the last_name column on every row
in the table (this is known as a full table scan). With an index the
database simply follows the B-tree data structure until the Smith
entry has been found; this is much less computationally expensive than
a full table scan.
Generally speaking the more unique values there are in a column, or the higher its cardinality What is cardinality in MySQL?, the more useful an index will be on that column.

MySQL Index + Query Processing

Assume I have this table:
create table table_a (
id int,
name varchar(25),
address varchar(25),
primary key (id)
) engine = innodb;
When I run this query:
select * from table_a where id >= 'x' and name = 'test';
How will MySQL process it? Will it pull all the id's first (assume 1000 rows) then apply the where clause name = 'test'?
Or while it looks for the ids, it is already applying the where clause at the same time?
As id is the PK (and no index on name) it will load all rows that satisfy the id based criterion into memory after which it will filter the resultset by the name criterion. Adding a composite index containing both fields would mean that it would only load the records that satisfy both criteria. Adding a separate single column index on the name field may not result in an index merge operation, in which case the index would have no effect.
Do you have indexes on either column? That may affect the execution plan. The other thing is one might cast the 'x'::int to ensure a numeric comparison instead of a string comparison.
For the best result, you should have a single index which includes both of the columns id and name.
In your case, I can't answer the affect of the primary index to that query. That depends on DBMS's and versions. If you really don't want to put more index (because more index means slow write and updates) just populate your table with like 10.000.000 random results, try it and see the effect.
you can compare the execution times by executing the query first when the id comes first in the where clause and then interchange and bring the name first. to see an example of mysql performance with indexes check this out http://www.mysqlperformanceblog.com/2006/06/02/indexes-in-mysql/
You can get information on how the query is processed by running EXPLAIN on the query.
If the idea is to optimize that query then you might want to add an index like:
alter table table_a add unique index name_id_idx (name, id);