System performance tuning

What is Performance Tuning
• Performance tuning can involve configuration
changes to software components.
• Performance tuning can also include tuning SQL
queries and tuning an applications underlying code
to cater for concurrency and to improve efficiency.

Common Problems in SQL Queries

Pagination
• Fetching lots of records can cause slow down
the speed of system but its necessary because
database is systematically organized or
structure repository of indexed information.

Pagination
Example:
Without pagination fetching all data
SELECT * FROM cats

Pagination
Two things you must do:
• Decide on the maximum number of database
rows that can be included in each page. You
may hard code this value, or (my preferred
method) you can define it in a variable so that
the value may be changed at runtime.

Pagination
• You then need to inform the user that other
'pages' are available and provide a mechanism
whereby the user is able to select a different
'page' of details. I currently use a set of
hyperlinks in a separate pagination area which
looks like this:

Pagination
How to do it :
1. Obtain the required page number.
- if(isset($_GET[‘pageno’])) {
$pageno = $_GET[‘pageno’];
} else {
$pageno = 1;
}

Pagination
2. Identify how many database rows are
available:
-$query = “SELECT count(1) FROM table
WHERE…”;
$result = mysql_query($query, $db);
$query_data = mysql_fetch_row($result);
$numrows = $query_data[0];

Pagination
3. Calculate number of $lastpage:
- $rows_per_page = 15;
$lastpage = ceil($numrows/$rows_per_page);
4.Ensure that $pageno is within range:
-$pageno = (int)$pageno;
If ($pageno > $lastpage) {
$pageno = $lastpage;
}
If ($pageno < 1 ){
$pageno = 1;
}

Pagination
5. Construct LIMIT clause:
- $limit = ‘LIMIT’ .($pageno - 1) *
$rows_per_page.’,’.$rows_per_page;
6. Issue the database query:
- $query = “SELECT * FROM table $limit”
$result = mysql_query($query,$db);

Pagination
7. Construct pagination hyperlinks:
- If ($pageno == 1) {
echo “PREV”;
} else {
echo " <a href='{$_SERVER['PHP_SELF']}?pageno=1'>FIRST</a> ";
$prevpage = $pageno-1;
echo " <a href='{$_SERVER['PHP_SELF']}?pageno=$prevpage'>PREV</a> ";
}

Pagination
if ($pageno == $lastpage) {
echo " NEXT";
} else {
$nextpage = $pageno+1;
echo " <a
href='{$_SERVER['PHP_SELF']}?pageno=$nextpage'>NEXT
</a> ";
echo " <a
href='{$_SERVER['PHP_SELF']}?pageno=$lastpage'>LAST<
/a> ";
}

Performance: N+1 Query Problem
• Select N+1 is a data access anti-pattern
where the database is accessed in a
suboptimal way.
• Detecting Select N+1 problem usually means
that the data fetch strategy of the application
can be optimized.

Example:
$test = new test();
$hats = $test->getHats();
$cats = [];
foreach ($hats as $value) {
$cats[] = $test->getCats($value['id']);
}

Assuming $hats() has an implementation that boils
to:
SELECT * FROM hats WHERE …
.. And $cats($hats_id) has an implementation like
this:
SELECT * FROM cats WHERE hats_id = ..

.. You will issue “N+1” queries when the code executes, Where
N is the number of cats:
SELECT * FROM cats WHERE ..
SELECT * FROM cats WHERE hats_id = 1
….

Solution :
Batching Queries
$cats=getCats();

That is issue these queries:
SELECT * FROM cats WHERE …
SELECT cats.id, cats.name, cats.created
FROM cats INNER JOIN hats
ON hats.id = cats.hats_id

• It is much faster to issue 1 query which
returns thousands results than to issue t
queries which each thousands return 1
result.

System performance tuning

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