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.
Related
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.
Is it useful for SELECT performance to set an index on a field that contains only distinct values?
eg:
order_id
--------
98317490
10928343
82931376
93438473
...
Is it useful for SELECT performance to set an index on a field that contains only distinct values?
That depends. An index is useful if you often search on this column:
WHERE column=value
WHERE column BETWEEN a AND b
The usefulness of an index is determined by its selectivity. For example, if your column contains a boolean, which is:
false in 99.9% of rows
true in 0.1% of rows
Then you can easily guess that using an index to find "true" values will be a huge boost relative to reading the entire table to search for them.
On the other hand, searching for "false" using an index will be slower than not using an index, since you're gonna read the whole table anyway, you might as well not bother to also process the index.
If values are all distinct, then selectivity is maximum, and index will be very useful. That is, assuming you actually search on that column!
An index that is never used only slows down updates.
Of course it is useful, as with all indexes - it is useful if you have select statements where you have this field on the WHERE clause.
Whether this field has distinct values or not doesn't really matter.
Note that if your field is marked as UNIQUE or PRIMARY KEY in the database, the database will technically already have an index for this field, so adding another index for it will not change anything.
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.
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.
I have already posted a question about this, but the situation has changed sufficiently to warrant a new one.
I have a MySQL table called aromaProducts in which there are 7 columns with the FULLTEXT index, and which has three records in it. When I make a query against it like:
SELECT * FROM aromaProducts WHERE MATCH (title) AGAINST ('chamomile');
I get the correct result. However, when I try adding a second field to search in, I get an error:
Can't find FULLTEXT index matching the column list
Every column on its own works fine. I have also explicitly adding WITH QUERY EXPANSION and the same thing.
I have another table, aromaProducts1, and instead of assigning FULLTEXT to the fields one at a time, I assigned it to all 7 at the moment of table creation. Against this table, no queries work. When examining the table structure, the difference is this:
The first table shows each field having its own FULLTEXT index, while the second has one index, named title (the first field to have it assigned), and it applies to all seven fields.
All columns that I have made FULLTEXT are either VARCHAR or TEXT datatypes. I have no clue what the problem is.
You have to create the fulltext index on all columns you are going to match
If you want to match on (col1,col2,col3) you have to create fulltext index on col1,col2 and col3. If you want to match on(col1,col2) you have to implement another fulltext index(on col1 and col2), you can't use that one on col1, col2 and col3
you have to create one fulltext index with several columns, not one for each