Does it group all possible results and then send back the results found within the given LIMIT?
This page from the MySQL manual explains the ways in which it optimizes queries that use LIMIT:
http://dev.mysql.com/doc/refman/5.1/en/limit-optimization.html
In short, it doesn't just do the naive thing, which would be to compute the entire result set and then send you the first N rows.
There are some things that will prevent optimizations, including using a HAVING clause and using SQL_CALC_FOUND_ROWS.
The LIMIT clause is only selection of the records returned - it has nothing to do with the value(s) returned from the query itself. For example, if you use LIMIT 10 on a query that only returns 5 rows - you'll only get 5 rows. If the query returned 11+ rows, you'd only get 10 rows. For a query using LIMIT to consistently return the same results, you need to specify an ORDER BY clause.
Think of the query occurring in the following steps:
Query without ORDER BY or LIMIT
ORDER BY criteria applied to query output from Step 1
LIMIT criteria applied to output from Steps 1 & 2
If there's no ORDER BY specified, the step is not performed.
Related
Can someone explain how construction group by + having + limit exactly work? MySQL query:
SELECT
id,
avg(sal)
FROM
StreamData
WHERE
...
GROUP BY
id
HAVING
avg(sal)>=10.0
AND avg(sal)<=50.0
LIMIT 100
Query without limit and having clauses executes for 7 seconds, with limit - instantly if condition covers a large amount of data or ~7 seconds otherwise.
Documentation says that limit executes after having which after group by, this means that query should always execute for ~7 seconds. Please help to figure out what is limited by LIMIT clause.
Using LIMIT 100 simply tells MySQL to return only the first 100 records from your result set. Assuming that you are measuring the query time as the round trip from Java, then one component of the query time is the network time needed to move the result set from MySQL across the network. This can take a considerable time for a large result set, and using LIMIT 100 should reduce this time to zero or near zero.
Things are logically applied in a certain pipeline in SQL:
Table expressions are generated and executed (FROM, JOIN)
Rows filtered (WHERE)
Projections and aggregations applied (column list, aggregates, GROUP BY)
Aggregations filtered (HAVING)
Results limited (LIMIT, OFFSET)
Now these may be composed into a different execution order by the planner if that is safe but you always get the proper data out if you think through them in this order.
So group by groups, then these are filtered with having, then the results of that are truncated.
As soon as MySQL has sent the required number of rows to the client,
it aborts the query unless you are using SQL_CALC_FOUND_ROWS. The
number of rows can then be retrieved with SELECT FOUND_ROWS(). See
Section 13.14, “Information Functions”.
http://dev.mysql.com/doc/refman/5.7/en/limit-optimization.html
This effectively means that if your table has a rather hefty number of rows, the server doesn't need to look at all of them. It can stop as soon as it has found a 100 because it knows that's all that you need.
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Find total number of results in mySQL query with offset+limit
I have a very complex sql query which returns results that are paginated. The problem is to get the total row count before LIMIT I have to run the sql query twice. The first time without the limit clause to get the total row count. The sql query is really complex and I think they must be a better way of doing this without running the query twice.
Luckily since MySQL 4.0.0 you can use SQL_CALC_FOUND_ROWS option in your query which will tell MySQL to count total number of rows disregarding LIMIT clause. You still need to execute a second query in order to retrieve row count, but it’s a simple query and not as complex as your query which retrieved the data.
Usage is pretty simple. In you main query you need to add SQL_CALC_FOUND_ROWS option just after SELECT and in second query you need to use FOUND_ROWS() function to get total number of rows. Queries would look like this:
SELECT SQL_CALC_FOUND_ROWS name, email FROM users WHERE name LIKE 'a%' LIMIT 10;
SELECT FOUND_ROWS();
The only limitation is that you must call second query immediately after the first one because SQL_CALC_FOUND_ROWS does not save number of rows anywhere.
Although this solution also requires two queries it’s much faster, as you execute the main query only once.
You can read more about SQL_CALC_FOUND_ROWS and FOUND_ROWS() in MySQL docs.
EDIT: You should note that in most cases running the query twice is actually faster than SQL_CALC_FOUND_ROWS. see here
EDIT 2019:
The SQL_CALC_FOUND_ROWS query modifier and accompanying FOUND_ROWS() function are deprecated as of MySQL 8.0.17 and will be removed in a future MySQL version.
https://dev.mysql.com/doc/refman/8.0/en/information-functions.html#function_found-rows
It's recommended to use COUNT instead
SELECT * FROM tbl_name WHERE id > 100 LIMIT 10;
SELECT COUNT(*) WHERE id > 100;
Im in a dilema on which one of these methods are most efficient.
Suppose you have a query joining multiple tables and querying thousand of records. Than, you gotta get the total to paginate throughout all these results.
Is it faster to?
1) Do a complete select (suppose you have to select 50's columns), count the rows and than run another query with limits? (Will the MySQL cache help this case already selecting all the columns you need on the first query used to count?)
2) First do the query using COUNT function and than do the query to select the results you need.
3) Instead of using MySQL COUNT function, do the query selecting the ID's for example and use the PHP function mysql_num_rows?
I think the number 2 is the best option, using MySQL built in COUNT function, but I know MySQL uses cache, so, selecting all the results on first query gonna be faster?
Thanks,
Have a look at Found_Rows()
A SELECT statement may include a LIMIT clause to restrict the number
of rows the server returns to the client. In some cases, it is
desirable to know how many rows the statement would have returned
without the LIMIT, but without running the statement again. To obtain
this row count, include a SQL_CALC_FOUND_ROWS option in the SELECT
statement, and then invoke FOUND_ROWS() afterward:
mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name WHERE id > 100 LIMIT 10;
mysql> SELECT FOUND_ROWS();
The second SELECT returns a number indicating how many rows the first SELECT`
would have returned had it been written without the LIMIT clause.
My guess is number 2, but the truth is that it will depend entirely on data size, tables, indexing, MySql version etc.
The only way of finding the answer to this is to try each one and measure how long they take. But like I say, my hunch would be number 2.
I am testing my database design under load and I need to retrieve only a fixed number of rows (5000)
I can specify a LIMIT to achieve this, however it seems that the query builds the result set of all rows that match and then returns only the number of rows specified in the limit. Is that how it is implemented?
Is there a for MySQL to read one row, read another one and basically stop when it retrieves the 5000th matching row?
MySQL is smart in that if you specify a LIMIT 5000 in your query, and it is possible to produce that result without generating the whole result set first, then it will not build the whole result.
For instance, the following query:
SELECT * FROM table ORDER BY column LIMIT 5000
This query will need to scan the whole table unless there is an index on column, in which case it does the smart thing and uses the index to find the rows with the smallest column.
SELECT * FROM `your_table` LIMIT 0, 5000
This will display the first 5000 results from the database.
SELECT * FROM `your_table` LIMIT 1001, 5000
This will show records from 1001 to 6000 (counting from 0).
Complexity of such query is O(LIMIT) (unless you specify order by).
It means that if 10000000 rows will match your query, and you specify limit equal to 5000, then the complexity will be O(5000).
#Jarosław Gomułka is right
If you use LIMIT with ORDER BY, MySQL ends the sorting as soon as it has found the first row_count rows of the sorted result, rather than sorting the entire result. If ordering is done by using an index, this is very fast. In either case, after the initial rows have been found, there is no need to sort any remainder of the result set, and MySQL does not do so.
if the set is not sorted it terminates the SELECT operation as soon as it's got enough rows to the result set.
The exact plan the query optimizer uses depends on your query (what fields are being selected, the LIMIT amount and whether there is an ORDER BY) and your table (keys, indexes, and number of rows in the table). Selecting an unindexed column and/or ordering by a non-key column is going to produce a different execution plan than selecting a column and ordering by the primary key column. The later will not even touch the table, and only process the number of rows specified in your LIMIT.
Each database defines its own way of limiting the result set size depends on the database you are using.
While the SQL:2008 specification defines a standard syntax for limiting a SQL query, MySQL 8 does not support it.
Therefore, on MySQL, you need to use the LIMIT clause to restrict the result set to the Top-N records:
SELECT
title
FROM
post
ORDER BY
id DESC
LIMIT 50
Notice that we are using an ORDER BY clause since, otherwise, there is no guarantee which are the first records to be included in the returning result set.
I have 4000 rows for example, and I define X limit.
The query stops after it finds X rows? or the query finds all the rows and then takes X rows from the found rows?
Thank you.
From MySQL Reference Manual:
If you use LIMIT row_count with ORDER BY, MySQL ends the sorting as
soon as it has found the first row_count rows of the sorted result,
rather than sorting the entire result. If ordering is done by using an
index, this is very fast. If a filesort must be done, all rows that
match the query without the LIMIT clause must be selected, and most or
all of them must be sorted, before it can be ascertained that the
first row_count rows have been found. In either case, after the
initial rows have been found, there is no need to sort any remainder
of the result set, and MySQL does not do so.
So it looks like it's possible that the entire result set is known before the LIMIT is applied. But MySQL will try everything it can not to do so. And you can help it by providing useful indexes that match your queries.
EDIT: Furthermore, if the set is not sorted it terminates the SELECT operation as soon as it's streamed enough rows to the result set.
SELECT * FROM your_table LIMIT 0, 10
This will display the first 10 results from the database.
SELECT * FROM your_table LIMIT 5, 5
This will show records 6, 7, 8, 9, and 10
It's like telling MySql; I want you to start counting from 5+1 or the 6th record, but Select only upto 5 records
I'm assuming you're thinking about MySQL, in which according to the documentation, the answer is it depends. If you're using a LIMIT (without a HAVING), then:
If you are selecting only a few rows with LIMIT, MySQL uses indexes
in some cases when normally it would prefer to do a full table scan.
As soon as MySQL has sent the required number of rows to the client, it aborts the query unless you are using SQL_CALC_FOUND_ROWS.
There are a few other cases which you should read about in the documentation.
Introduction to MySQL LIMIT clause
The following illustrates the LIMIT clause syntax with two arguments:
SELECT
select_list
FROM
table_name
LIMIT [offset,] row_count;
The offset specifies the offset of the first row to return. The offset of the first row is 0, not 1.
The row_count specifies the maximum number of rows to return.
The following picture illustrates the LIMIT clause:
Therefore, these two clauses are equivalent:
> LIMIT row_count;
> LIMIT 0 , row_count;
The following picture illustrates the evaluation order of the LIMIT clause in the SELECT statement:
It stops after it found the number of rows specified in the LIMIT clause. This can be verified with a large amount of data. It retrieves the result in a time that is not possible if it is getting all the rows of the table and filtering after that.
If you are using MS SQL Server, then you can write it as given below.
Select TOP [x]
*
From MyTable
Hope it helps.
Vamyip