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Database ScalabilitySweep the Query Log & Keep it Clean

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What Is Scalability?

Webserver tier - Apache, Nginx

Object caches - Memcache

Search servers - Sphinx

Queueing servers - SQS, RabbitMQ

3

Why is DB hard to Scale?

Relational Databases

Single authoritative master

Scaling a write db is hard

Durability

SQL code weight non-obvious

4

ORM Advantages

Middleware, multi-database support

Reusable code, encapsulation & abstraction

Time saving - auto generated model objects

Built for wide audience, reduce need for SQL

Sacrifice a bit of efficiency for code agility

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ORMs + ScalabilityNegates standard optimization techniques

Difficult to divorce after you are wedded

Limit query tuning at the bare metal

Wide audience not high-scale applications

Bad at deducing JOINs

Fetch 20 cols when you need only 2?

“death by a thousand queries” - Laurie Voss

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ORM ExampleEx. from http://mattiasgeniar.be

<?php

// First, get all the companies in your database

$companies = $this->getAllCompanies();

$totalValue = 0;

foreach ($companies as $company) {

// For each company there is, retrieve the total value of all their orders

$orders = $company->getOrders();

foreach ($orders as $order) {

$totalValue += (float) $order->getValue();

}

 

echo "This company made us ". $totalValue ." euro already.";

}

?>

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ORM Example<?php

// First, get all the companies in your database

SELECT * FROM "company";

$totalValue = 0;

foreach (...) {

// For each company there is, retrieve the total value of all their orders

SELECT * FROM "order" WHERE companyid = $companyid;

foreach (...) {

$totalValue += $row->value;

}

 

echo "This company made us ". $totalValue ." euro already.";

}

?>

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ORM Example as SQL

SELECT SUM(o.VALUE) AS TotalValue, c.name AS CompanyName

FROM company AS c

LEFT JOIN "order" AS o ON o.companyid = c.companyid

GROUP BY o.companyid;

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ORM Articles

ORM is an anti-pattern - Laurie Voss

Bad ORM is infinitely worse than bad SQL - Mattias Geniar

Case against ORM frameworks - Todd Hoff

5 things toxic to scalability - Sean Hull

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Database Tuning Process

Identify heavy queries in slow log

Find queries in code

Rewrite, run explain & profile

Benchmark with large dataset

Deploy fixed query

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Being Reactive?

Sweep out the slow query log

Rewrite queries

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Being Proactive

Keep slow query log quiet

With new code deploys, new slow queries will pop up

Tune early, tune often

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Use Profiling

set profiling=1;

<run query>

show profile for query 1;

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Enable Session Profiling(sean@localhost:mysql.sock) [sakila]> set profiling = 1;

Query OK, 0 rows affected (0.00 sec)

(sean@localhost:mysql.sock) [sakila]> select customer_id, last_name from customer where customer_id = 5;

+-------------+-----------+

| customer_id | last_name |

+-------------+-----------+

| 5 | BROWN |

+-------------+-----------+

1 row in set (0.00 sec)

(sean@localhost:mysql.sock) [sakila]> select SQL_NO_CACHE customer_id, last_name from customer where customer_id = 5;

+-------------+-----------+

| customer_id | last_name |

+-------------+-----------+

| 5 | BROWN |

+-------------+-----------+

1 row in set (0.00 sec)

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Anatomy of a Profile(sean@localhost:mysql.sock) [sakila]> show profiles;

+----------+------------+--------------------------------------------------------------------------------+

| Query_ID | Duration | Query |

+----------+------------+--------------------------------------------------------------------------------+

| 1 | 0.00015500 | select customer_id, last_name from customer where customer_id = 5 |

| 2 | 0.00054700 | select SQL_NO_CACHE customer_id, last_name from customer where customer_id = 5 |

+----------+------------+--------------------------------------------------------------------------------+

2 rows in set (0.00 sec)

(sean@localhost:mysql.sock) [sakila]> show profile for query 1;

+--------------------------------+----------+

| Status | Duration |

+--------------------------------+----------+

| starting | 0.000062 |

| checking query cache for query | 0.000020 |

| checking privileges on cached | 0.000019 |

| sending cached result to clien | 0.000033 |

| logging slow query | 0.000010 |

| cleaning up | 0.000011 |

+--------------------------------+----------+

6 rows in set (0.00 sec)

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Non Cached Query Profile

(sean@localhost:mysql.sock) [sakila]> show profile for query 2;

+--------------------------------+----------+

| Status | Duration |

+--------------------------------+----------+

| starting | 0.000052 |

| checking query cache for query | 0.000103 |

| Opening tables | 0.000025 |

| System lock | 0.000014 |

| Table lock | 0.000022 |

| init | 0.000043 |

| optimizing | 0.000022 |

| statistics | 0.000095 |

| preparing | 0.000030 |

| executing | 0.000011 |

| Sending data | 0.000045 |

| end | 0.000013 |

| end | 0.000010 |

| query end | 0.000012 |

| freeing items | 0.000015 |

| closing tables | 0.000015 |

| logging slow query | 0.000010 |

| cleaning up | 0.000010 |

+--------------------------------+----------+

18 rows in set (0.00 sec)

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Single Column Index

create index d_idx on a (d);

select email from a where d=99;

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Multi-column Index

create index def_idx on a (d, e, f);

SELECT email FROM a WHERE d=1 and e=2 and f=3;

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Left-most Prefix Index

create index abc_idx on a (d, e, f);

SELECT name FROM a where d = 1 and e = 2;

SELECT name FROM a WHERE d=1 and e=2 and f=3;

SELECT name FROM a WHERE e=2;

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Covering Index

create index cover_id on (id, name);

SELECT name FROM a WHERE id = 5;

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Use EXPLAIN

Illustrates database engine’s path to the data

Shows logical & physical I/Os

Shows sorting, temp tables, index usage

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EXPLAIN - no index

(sean@localhost:mysql.sock) [sakila]> explain select last_name from customer where email = 'ELIZABETH.BROWN@sakilacustomer.org';

+----+-------------+----------+------+---------------+------+---------+------+------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------+------+---------------+------+---------+------+------+-------------+

| 1 | SIMPLE | customer | ALL | NULL | NULL | NULL | NULL | 541 | Using where |

+----+-------------+----------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

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EXPLAIN - basic index

(sean@localhost:mysql.sock) [sakila]> create index cust_email on customer(email);

(sean@localhost:mysql.sock) [sakila]> explain select last_name from customer where email = 'ELIZABETH.BROWN@sakilacustomer.org';

+----+-------------+----------+------+---------------+------------+---------+-------+------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------+------+---------------+------------+---------+-------+------+-------------+

| 1 | SIMPLE | customer | ref | cust_email | cust_email | 153 | const | 1 | Using where |

+----+-------------+----------+------+---------------+------------+---------+-------+------+-------------+

1 row in set (0.02 sec)

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EXPLAIN - covering index

(sean@localhost:mysql.sock) [sakila]> create index cust_email_ln on customer(email, last_name);

(sean@localhost:mysql.sock) [sakila]> explain select last_name from customer where email = 'ELIZABETH.BROWN@sakilacustomer.org';

+----+-------------+----------+------+---------------+---------------+---------+-------+------+--------------------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------+------+---------------+---------------+---------+-------+------+--------------------------+

| 1 | SIMPLE | customer | ref | cust_email_ln | cust_email_ln | 153 | const | 1 | Using where; Using index |

+----+-------------+----------+------+---------------+---------------+---------+-------+------+--------------------------+

1 row in set (0.00 sec)

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Test With Large Datasets

As data grows, indexing more crucial

Test with million row tables

Slow desktop or test boxes?

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Can Devops Help?

Teamwork - way of working

Less siloing of departments

DBAs helping identify queries

DBAs helping rewrite queries

Devs on top of new code deploys

Clean slow-log as new queries popup

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About Sean Hull

shull@iheavy.com

Join 8000 & please follow me: @hullsean

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