I am running this query
EXPLAIN SELECT id, timestamp from foo where id IN (23,67,78,90) order by ASC
here id is indexed. But then too when I am running Explain I am getting this in Using where;Using Index in Extra
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | foo | range | PRIMARY | PRIMARY | 8 | NULL | 12 | Using where;Using Index|
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
But when I am running this same query with single id nothing is in Extra its working as expected in the case of index
EXPLAIN SELECT id, timestamp from foo where id = 23`
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table| type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | foo | range | PRIMARY | PRIMARY | 8 | NULL | 1 | |
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
I think something wrong with IN. Can anyone tell me the way to optimize it ?
As per my knowledge,
IN query will take more time than Single "=". If "IN" have single value then it is equal to single "=" query.
Because it is MULTIPLE OR conditions of "=" OR "=".
id will match with every 4 values in "IN" array.
Only way to optimise is to have index.
Update:
If the Extra column also says Using where, it means the index is being used to perform lookups of key values. Without Using where, the optimizer may be reading the index to avoid reading data rows but not using it for lookups. For example, if the index is a covering index for the query, the optimizer may scan it without using it for lookups. See explanation
Related
I have the following MYSQL query:
SELECT statusdate,consigneenamee,weight,productcode,
pieces,statustime,statuscode,statusdesc,generatingiata,
shipmentdate,shipmenttime,consigneeairportcode,signatory
FROM notes
where (shipperref='180908184' OR shipperref='180908184'
OR shipperref='180908184' OR shipperref='180908184 '
OR shipperref like 'P_L_%180908184')
order by edicheckpointdate asc, edicheckpointtime asc;
I added an index to speed up this query using the MYSQL Workbench but when executing the EXPLAIN command, it still does not show the key and shows as NULL:
+----+-------------+---------------+------+---------------+------+---------+------+---------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+---------------+------+---------+------+---------+-----------------------------+
| 1 | SIMPLE | dhltracking_2 | ALL | index2 | NULL | NULL | NULL | 3920874 | Using where; Using filesort |
+----+-------------+---------------+------+---------------+------+---------+------+---------+-----------------------------+
Any reason why this is happening and how I can speed up this query?
My Index:
You have LIKE statement in your query and I think that your index spans on more than 20-30% of table rows (or more..) and that's why MySQL can ignore it for performance reasons.
My proposal:
Add FORCE INDEX as #Solarflare proposes
Use FULLTEXT index (works on CHAR and VARCHAR also) and use MATCH ... AGAINST search (https://dev.mysql.com/doc/refman/8.0/en/fulltext-boolean.html)
In a MySQL db I have a table that only has 2 columns, for all intents and purposes: a key hash and a value. Both are INTEGER type. The hash column will have a large number of duplicates (worst case expect ~80k dup for each hash, not possible to make unique due to small hash preimage), and the table contains on the order of 100 billion rows.
Right now I have the hash column indexed (CREATE INDEX idx_hash ON table(hash)); however lookups are very slow. Something like SELECT value FROM table WHERE hash=123 LIMIT 50 will take minutes if not hours, while a similar select on a similar sized table on a primary key column will finish in a jiffy on the same machine.
So my question is how do I optimize for lookup in this case? Is sub-linear time SELECT possible on index columns? This table will be mostly read-only, rebuilding it is possible but will take a long time, so I'd like to gather some information and do it correctly.
EXPLAIN says:
+----+-------------+----------------+------------+------+---------------+------+---------+------+-----------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-----------+----------+-------------+
| 1 | SIMPLE | partial_lookup | NULL | ALL | NULL | NULL | NULL | NULL | 100401571 | 10.00 | Using where |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-----------+----------+-------------+
ANALYZE:
+--------------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+--------------------+---------+----------+----------+
| partial_lookup | analyze | status | OK |
+--------------------+---------+----------+----------+
1 row in set (1.47 sec)
I am executing most of the queries based on the time. So i created index for the created time. But , The index only works , If I select the indexed columns only. Is mysql index is dependant the selected columns?.
My Assumption On Index
I thought index is like a telephone dictionary index page. Ex: If i want to find "Mark" . Index page shows which page character "M" starts in the directory. I think as same as the mysql works.
Table
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| ID | int(11) | NO | PRI | NULL | auto_increment |
| Name | varchar(100) | YES | | NULL | |
| OPERATION | varchar(100) | YES | | NULL | |
| PID | int(11) | YES | | NULL | |
| CREATED_TIME | bigint(20) | YES | | NULL | |
+--------------+--------------+------+-----+---------+----------------+
Indexes On the table.
+-----------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| IndexTest | 0 | PRIMARY | 1 | ID | A | 10261 | NULL | NULL | | BTREE | | |
| IndexTest | 1 | t_dx | 1 | CREATED_TIME | A | 410 | NULL | NULL | YES | BTREE | | |
+-----------+------------+----------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
Queries Using Indexes:
explain select * from IndexTest where ID < 5;
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | IndexTest | range | PRIMARY | PRIMARY | 4 | NULL | 4 | Using where |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
explain select CREATED_TIME from IndexTest where CREATED_TIME > UNIX_TIMESTAMP(CURRENT_DATE())*1000;
+----+-------------+-----------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+------+---------+------+------+--------------------------+
| 1 | SIMPLE | IndexTest | range | t_dx | t_dx | 9 | NULL | 5248 | Using where; Using index |
+----+-------------+-----------+-------+---------------+------+---------+------+------+--------------------------+
Queries Not using Indexes
explain select count(distinct(PID)) from IndexTest where CREATED_TIME > UNIX_TIMESTAMP(CURRENT_DATE())*1000;
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | IndexTest | ALL | t_dx | NULL | NULL | NULL | 10261 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
explain select PID from IndexTest where CREATED_TIME > UNIX_TIMESTAMP(CURRENT_DATE())*1000;
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | IndexTest | ALL | t_dx | NULL | NULL | NULL | 10261 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+
Short answer: No.
Whether indexes are used depends on the expresion in your WHERE clause, JOINs etc, but not on the columns you select.
But no rule without an exception (or actually a long list of those):
Long answer: Usually not
There are a number of factors used by the MySQL Optimizer in order to determine whether it should use an index.
The optimizer may decide to ignore an index if...
another (otherwise non-optimal) saves it from accessing the table data at all
it fails to understand that an expression is a constant
its estimates suggest it will return the full table anyway
if its use will cause the creation of a temporary file
... and tons of other reasons, some of which seem not to be documented anywhere
Sometimes the choices made by said optimizer are... erm... lets call them sub-optimal. Now what do you do in those cases?
You can help the optimizer by doing an OPTIMIZE TABLE and/or ANALYZE TABLE. That is easy to do, and sometimes helps.
You can make it use a certain index with the USE INDEX(indexname) or FORCE INDEX(indexname) syntax
You can make it ignore a certain index with the IGNORE INDEX(indexname) syntax
More details on Index Hints, Optimize Table and Analyze Table on the MySQL documentation website.
Actually, it makes no difference wether you select the column or not. Indexes are used for lookups, meaning for reducing really fast the number of records you need retrieved. That makes it usually useful in situations where: you have joins, you have where conditions. Also indexes help alot in ordering.
Updating and deleting can be sped up quite alot using indexes on the where conditions as well.
As an example:
table: id int pk ai, col1 ... indexed, col2 ...
select * from table -> does not use a index
select id from table where col1 = something -> uses the col1 index although it is not selected.
Looking at the second query, mysql does a lookup in the index, locates the records, then in this case stops and delivers (both id and col1 have index and id happens to be pk, so no need for a secondary lookup).
Situation changes a little in this case:
select col2 from table where col1 = something
This will make internally 2 lookups: 1 for the condition, and 1 on the pk for delivering the col2 data. Please notice that again, you don't need to select the col1 column to use the index.
Getting back to your query, the problem lies with: UNIX_TIMESTAMP(CURRENT_DATE())*1000;
If you remove that, your index will be used for lookups.
Is mysql index is dependant the selected columns?.
Yes, absolutely.
For example:
MySQL cannot use the index to perform lookups if the columns do not form a leftmost
prefix of the index. Suppose that you have the SELECT statements shown here:
SELECT * FROM tbl_name WHERE col1=val1;
SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;
SELECT * FROM tbl_name WHERE col2=val2;
SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;
If an index exists on (col1, col2, col3), only the first two queries use the index.
The third and fourth queries do involve indexed columns, but (col2) and (col2, col3)
are not leftmost prefixes of (col1, col2, col3).
Have a read through the extensive documentation.
for mysql query , the answer is yes, but not all
the query:
explain select * from IndexTest where ID < 5;
use the table cluster index if you use innodb, its table's primary key, so it use primary for query
the second query:
select CREATED_TIME from IndexTest where CREATED_TIME >
UNIX_TIMESTAMP(CURRENT_DATE())*1000;
this one is just fetch the index column that mysql does not need to fetch data from table but just index, so your explain result got "Using Index"
the query:
select count(distinct(PID)) from IndexTest where CREATED_TIME >
UNIX_TIMESTAMP(CURRENT_DATE())*1000;
it look like this
select PID from IndexTest where
CREATE_TIME>UNIX_TIMESTAMP(CURRENT_DATE())*1000 group by PID
mysql can use index to fetch data from database also, but mysql thinks this query it no need to use index to fetch data, because of the where condition filter, mysql thinks that use index fetch data is more expensive than scan all table, you can use force index also
the same reason for your last query
hopp this answer can help you
indexing helps speed the search for that particular column and associated data rather than the table data. So you have to include the indexed column to speed up select.
Please consider following schema
CREATE table articles (
id Int UNSIGNED NOT NULL AUTO_INCREMENT,
cat_id Int UNSIGNED NOT NULL,
status Int UNSIGNED NOT NULL,
date_added Datetime,
Primary Key (id)) ENGINE = InnoDB;
CREATE INDEX cat_list_INX ON articles (cat_id, status, date_added);
CREATE INDEX categories_list_INX ON articles (cat_id, status);
I have written following two queries which can be completely satisfied by the above two indicies but MySQL is putting where in extra column.
mysql> EXPLAIN SELECT cat_id FROM articles USE INDEX (cat_list_INX) WHERE cat_id=158 AND status=2 ORDER BY date_added DESC LIMIT 500, 5;
+----+-------------+----------+------+---------------+--------------+---------+-------------+-------+--------------------------+
| id | select_type | table ref | | type | possible_keys | key | key_len | rows | Extra |
+----+-------------+----------+------+---------------+--------------+---------+-------------+-------+--------------------------+
| 1 | SIMPLE | articles | ref | cat_list_INX | cat_list_INX | 5 | const,const | 50698 | Using where; Using index |
+----+-------------+----------+------+---------------+--------------+---------+-------------+-------+--------------------------+
mysql> EXPLAIN SELECT cat_id FROM articles USE INDEX (categories_list_INX) WHERE cat_id=158 AND status=2;
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+-------+--------------------------+
| id | select_type | tab key |le | type | possible_keys | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+-------+--------------------------+
| 1 | SIMPLE | articles | ref | categories_list_INX | categories_list_INX | 5 | const,const | 52710 | Using where; Using index |
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+-------+--------------------------+
As far as I know where requires an additional disk seek. Why it's not just using index?
The first query is filtering records at the mysql level outside of the storage engine because of your "ORDER BY" clause using date_added field.
This can be mitigated by moving the date_added field first in the index like this
CREATE INDEX cat_list_INX ON articles (date_added, cat_id, status);
The 2nd query - my version of mysql is not showing a "Using where" - I would not expect to either - maybe its because I have no records.
mysql> EXPLAIN SELECT cat_id FROM articles USE INDEX (categories_list_INX) WHERE cat_id=158 AND status=2;
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+------+-------------+
| 1 | SIMPLE | articles | ref | categories_list_INX | categories_list_INX | 8 | const,const | 1 | Using index |
+----+-------------+----------+------+---------------------+---------------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)
Extra column info from High Performance MySQL:
Using Index: This indicates that MySQL will use a covering index to avoid accessing the table. Don't confuse covering indexes with the index access type.
Using Where: This means the MySQL server will post-filter rows after the storage engine retrieves them. Many WHERE conditions that involve columns in an index can be checked by the storage engine when (and if) it reads the index, so not all queries with a WHERE clause will show "Using where". Sometimes the presence of "Using where" is a hint that the query can benefit from different indexing.
Table structure:
+-------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+----------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| total | int(11) | YES | | NULL | |
| thedatetime | datetime | YES | MUL | NULL | |
+-------------+----------+------+-----+---------+----------------+
Total rows: 137967
mysql> explain select * from out where thedatetime <= NOW();
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+
| 1 | SIMPLE | out | ALL | thedatetime | NULL | NULL | NULL | 137967 | Using where |
+----+-------------+-------------+------+---------------+------+---------+------+--------+-------------+
The real query is much more longer with more table joins, the point is, I can't get the table to use the datetime index. This is going to be hard for me if I want to select all data until certain date. However, I noticed that I can get MySQL to use the index if I select a smaller subset of data.
mysql> explain select * from out where thedatetime <= '2008-01-01';
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+
| 1 | SIMPLE | out | range | thedatetime | thedatetime | 9 | NULL | 15826 | Using where |
+----+-------------+-------------+-------+---------------+-------------+---------+------+-------+-------------+
mysql> select count(*) from out where thedatetime <= '2008-01-01';
+----------+
| count(*) |
+----------+
| 15990 |
+----------+
So, what can I do to make sure MySQL will use the index no matter what date that I put?
There are two things in play here -
Index is not selective enough - if the index covers more than approx. 30% of the rows, MySQL will decide a full table scan is more efficient. When you contract the range the index kicks in.
One index per table in a join
The real query is much more longer
with more table joins, the point is ...
The point is exactly because it has joins that it probably can't use that index. MySQL can use one index per table in a join (unless it qualifies for an index-merge optimization). If the primary key is already used for the join, thedatetime won't be used. In order to use it, you need to create a multi-column index on the join key + thedatetime index, in the correct order.
Check the EXPLAIN of the actual query to see which key MySQL uses for the join. Modify that index to include the thedatetime column as well, or create a new multi-column index from both (depending on what you use the join key for).
Everything works as it is supposed to. :)
Indexes are there to speed up retrieval. They do it using index lookups.
In you first query the index is not used because you are retrieving ALL rows, and in this case using index is slower (lookup index, get row, lookup index, get row... x number of rows is slower then get all rows == table scan)
In the second query you are retrieving only a portion of the data and in this case table scan is much slower.
The job of the optimizer is to use statistics that RDBMS keeps on the index to determine the best plan. In first case index was considered, but planner (correctly) threw it away.
EDIT
You might want to read something like this to get some concepts and keywords regarding mysql query planner.