MySQL Data - Best way to implement paging

Question

My iPhone app connects to my PHP web service to retrieve data from a MySQL database  A request can return 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 ads from my database  Now how can I request for the next 20 ads

User · Answer

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.

User · Answer

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

User · Answer

Query 1  SELECT   FROM yourtable WHERE id  gt  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

User · Answer

This tutorial shows a great way to do pagination  Efficient Pagination Using MySQL  In short  avoid to use OFFSET or large LIMIT

User · Answer

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

User · Answer

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

User · Answer

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

[mysql] MySQL Data - Best way to implement paging?

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