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
Related
If I have a mysql limited query:
SELECT * FROM my_table WHERE date > '2020-12-12' LIMIT 1,16;
Is there a faster way to check and see how many results are left after my limit?
I was trying to do a count with limit, but that wasn't working, i.e.
SELECT count(ID) AS count FROM my_table WHERE date > '2020-12-12' LIMIT 16,32;
The ultimate goal here is just to determine if there ARE any other rows to be had beyond the current result set, so if there is another faster way to do this that would be fine too.
It's best to do this by counting the rows:
SELECT count(*) AS count FROM my_table WHERE date > '2020-12-12'
That tells you how many total rows match the condition. Then you can compare that to the size of the result you got with your query using LIMIT. It's just arithmetic.
Past versions of MySQL had a function FOUND_ROWS() which would report how many rows would have matched if you didn't use LIMIT. But it turns out this had worse performance than running two queries, one to count rows and one to do your limit. So they deprecated this feature.
For details read:
https://www.percona.com/blog/2007/08/28/to-sql_calc_found_rows-or-not-to-sql_calc_found_rows/
https://dev.mysql.com/worklog/task/?id=12615
(You probably want OFFSET 0, not 1.)
It's simple to test whether there ARE more rows. Assuming you want 16 rows, use 1 more:
SELECT ... WHERE ... ORDER BY ... LIMIT 0,17
Then programmatically see whether it returned only 16 rows (no more available) or 17 (there ARE more).
Because it is piggybacking on the fetch you are already doing and not doing much extra work, it is very efficient.
The second 'page' would use LIMIT 16, 17; 3rd: LIMIT 32,17, etc. Each time, you are potentially getting and tossing an extra row.
I discuss this and other tricks where I point out the evils of OFFSET: Pagination
COUNT(x) checks x for being NOT NULL. This is [usually] unnecessary. The pattern COUNT(*) (or COUNT(1)) simply counts rows; the * or 1 has no significance.
SELECT COUNT(*) FROM t is not free. It will actually do a full index scan, which is slow for a large table. WHERE and ORDER BY are likely to add to that slowness. LIMIT is useless since the result is always 1 row. (That is, the LIMIT is applied to the result, not to the counting.)
I want to achieve the below 2 scenarios in a single query.
(Note- This is just for reference, the actual query is different)
1. SELECT * FROM CUSTOMER.CUSTOMER LIMIT :startingRow, rowsCount; //WITH LIMIT
2. SELECT * FROM CUSTOMER.CUSTOMER; // NO LIMIT
Is it possible to write a single conditional query for this?
If I pass starting row and rows count params it should go for 1st condition and if no input params are passed, it should give me all records from a table.
The MySQL manual gives a tip for this:
https://dev.mysql.com/doc/refman/en/select.html
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
The very large value used in this example is 264-1, or the greatest value of BIGINT UNSIGNED. There are certainly a smaller number of rows in your table.
In your case, you could use 0 as the default offset and a very large value like that as the default limit.
Speaking for myself, I would just run two different queries. One with a LIMIT clause, and the other with no LIMIT clause. Use some kind of if/then/else structure in your client code to determine which query to run, based on whether the function has specified the limit parameters or not.
My iPhone application connects to my PHP web service to retrieve data from a MySQL database, a request can return up to 500 results.
What is the best way to implement paging and retrieve 20 items at a time?
Let's say I receive the first 20 entries from my database, how can I now request the next 20 entries?
From the MySQL documentation:
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows
In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.
For 500 records efficiency is probably not an issue, but if you have millions of records then it can be advantageous to use a WHERE clause to select the next page:
SELECT *
FROM yourtable
WHERE id > 234374
ORDER BY id
LIMIT 20
The "234374" here is the id of the last record from the prevous page you viewed.
This will enable an index on id to be used to find the first record. If you use LIMIT offset, 20 you could find that it gets slower and slower as you page towards the end. As I said, it probably won't matter if you have only 200 records, but it can make a difference with larger result sets.
Another advantage of this approach is that if the data changes between the calls you won't miss records or get a repeated record. This is because adding or removing a row means that the offset of all the rows after it changes. In your case it's probably not important - I guess your pool of adverts doesn't change too often and anyway no-one would notice if they get the same ad twice in a row - but if you're looking for the "best way" then this is another thing to keep in mind when choosing which approach to use.
If you do wish to use LIMIT with an offset (and this is necessary if a user navigates directly to page 10000 instead of paging through pages one by one) then you could read this article about late row lookups to improve performance of LIMIT with a large offset.
Define OFFSET for the query. For example
page 1 - (records 01-10): offset = 0, limit=10;
page 2 - (records 11-20) offset = 10, limit =10;
and use the following query :
SELECT column FROM table LIMIT {someLimit} OFFSET {someOffset};
example for page 2:
SELECT column FROM table
LIMIT 10 OFFSET 10;
There's literature about it:
Optimized Pagination using MySQL, making the difference between counting the total amount of rows, and pagination.
Efficient Pagination Using MySQL, by Yahoo Inc. in the Percona Performance Conference 2009. The Percona MySQL team provides it also as a Youtube video: Efficient Pagination Using MySQL (video),
The main problem happens with the usage of large OFFSETs. They avoid using OFFSET with a variety of techniques, ranging from id range selections in the WHERE clause, to some kind of caching or pre-computing pages.
There are suggested solutions at Use the INDEX, Luke:
"Paging Through Results".
"Pagination done the right way".
This tutorial shows a great way to do pagination.
Efficient Pagination Using MySQL
In short, avoid to use OFFSET or large LIMIT
you can also do
SELECT SQL_CALC_FOUND_ROWS * FROM tbl limit 0, 20
The row count of the select statement (without the limit) is captured in the same select statement so that you don't need to query the table size again.
You get the row count using SELECT FOUND_ROWS();
Query 1: SELECT * FROM yourtable WHERE id > 0 ORDER BY id LIMIT 500
Query 2: SELECT * FROM tbl LIMIT 0,500;
Query 1 run faster with small or medium records, if number of records equal 5,000 or higher, the result are similar.
Result for 500 records:
Query1 take 9.9999904632568 milliseconds
Query2 take 19.999980926514 milliseconds
Result for 8,000 records:
Query1 take 129.99987602234 milliseconds
Query2 take 160.00008583069 milliseconds
Here's how I'm solving this problem using node.js and a MySQL database.
First, lets declare our variables!
const
Key = payload.Key,
NumberToShowPerPage = payload.NumberToShowPerPage,
Offset = payload.PageNumber * NumberToShowPerPage;
NumberToShowPerPage is obvious, but the offset is the page number.
Now the SQL query...
pool.query("SELECT * FROM TableName WHERE Key = ? ORDER BY CreatedDate DESC LIMIT ? OFFSET ?", [Key, NumberToShowPerPage, Offset], (err, rows, fields) => {}));
I'll break this down a bit.
Pool, is a pool of MySQL connections. It comes from mysql node package module. You can create a connection pool using mysql.createPool.
The ?s are replaced by the variables in the array [PageKey, NumberToShow, Offset] in sequential order. This is done to prevent SQL injection.
See at the end were the () => {} is? That's an arrow function. Whatever you want to do with the data, put that logic between the braces.
Key = ? is something I'm using to select a certain foreign key. You would likely remove that if you don't use foreign key constraints.
Hope this helps.
If you are wanting to do this in a stored procedure you can try this
SELECT * FROM tbl limit 0, 20.
Unfortunately using formulas doesn't work so you can you execute a prepared statement or just give the begin and end values to the procedure.
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.
In MySQL, how can I retrieve ALL rows in a table, starting from row X? For example, starting from row 6:
LIMIT 5,0
This returns nothing, so I tried this:
LIMIT 5,ALL
Still no results (sql error).
I'm not looking for pagination functionality, just retrieving all rows starting from a particular row. LIMIT 5,2000 seems like overkill to me. Somehow Google doesn't seem to get me some answers. Hope you can help.
Thanks
According to the documentation:
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95, 18446744073709551615;
This is the maximum rows a MyISAM table can hold, 2^64-1.
There is a limit of 2^32 (~4.295E+09) rows in a MyISAM table. If you build MySQL with the --with-big-tables option, the row limitation is increased to (2^32)^2 (1.844E+19) rows. See Section 2.16.2, “Typical configure Options”. Binary distributions for Unix and Linux are built with this option.
If you're looking to get the last x number of rows, the easiest thing to do is SORT DESC and LIMIT to the first x rows. Granted, the SORT will slow your query down. But if you're opposed to setting an arbitrarily large number as the second LIMIT arg, then that's the way to do it.
The only solution I am aware of currently is to do as you say and give a ridiculously high number as the second argument to LIMIT. I do not believe there is any difference in performance to specifying a low number or a high number, mysql will simply stop returning rows at the end of the result set, or when it hits your limit.
I think you don't need to enter max value for select all by LIMIT. It is enough to find count of table and then use it as max LIMIT.
The next query should work too, and is in my opinion more effective...
SELECT * FROM mytbl WHERE id != 1 ORDER BY id asc
By ordering the query will find the id imediately and skip this one, so the next rows he won't check anymore whether the id = 1.