Random Musings of an Insane Mind

While I’ve used memcached for a number of things where MySQL’s query cache just isn’t quick enough, the nature of a key-value store without unions didn’t work for this particular project. While it would have been easy enough to run memcached alongside Redis, two software stacks to solve the same problem wasn’t appealing.

What I’ve come up with will work for either memcached or Redis and the theory is simple:

Create a unique key for your counter, increment the key, store the key in a list. Have a separate process iterate through the list, write the summarized data to your database, reinsert key into list if it is for the current hour.

Using r as our Redis object, the pseudocode looks like:

    dayhour_key = time.strftime('%Y%m%d%H', time.localtime())
    r.sinterstore('processlog', ['log'])
    numitems = r.scard('processlog')    # return number of items in our set 'log'

    for loop in range(0, numitems):
        logkey = r.spop('processlog')     # grab an item from our set 'log' and delete it from the set
        (table,dayhour) = logkey.split(':')
        count = r.get(logkey)     # get the count from our key
        if count == 0:
            # if the count is 0, delete the key (leftover from same hour decrement)
            r.delete(logkey)
        else:
            if dayhour < dayhour_key:
                # do our atomic update/insert incrementing table by count
                r.srem('log', logkey)
                r.delete(logkey)             # delete our key, it is not from the current hour
            else:
                # if we are processing the current hour, we must decrement by count in case 
                # another process modified the value while we were working
                r.decrby(logkey, count)    # decrement the key by count
                r.sadd('log', logkey)          # add the key to our set for processing

The concept is to use a key that is as granular as the data you want to keep. In this case we append a datehour stamp of yyyymmddHH (year, month, day, hour) to our unique id and end up with a key of stat:id:datehour. We use stat: to signify that the entry is for Statistics. For Zone 1 we end up with a key of stat:1:2010102314 (assuming 2pm) which is incremented and added to our 'log' set. When our log daemon runs in the current hour, we decrement the key by the count, and readd it to our set. If the log daemon runs on something in the past, we know that it cannot receive any updates so we are free to delete the key. Since we have used pop to remove the item from the list, any data prior to the current hour is automatically removed from the set, but, we need to add any key from the current hour.

We decrement the key by the count in the current hour just in case something else has updated that key while we were working. If in the next hour the count is 0, our routine skips it, but, still needs to delete the key.

In preliminary testing, we've saved roughly 280 transactions per second and stats are rarely more than a minute or two behind realtime. It also allowed us to move from daily to hourly statistics. The same theory could be applied to per-minute statistics as well.

While debugging an issue with an application that relies heavily on MySQL, an issue was brought up regarding the cardinality of the keys selected, and, the order in which the keys were indexed. With any relational database, in order to get the fastest performance, your query should reduce the result set as quickly as possible. Your data should have a high cardinality or variation in the data so that the B-Tree (or R-Tree) is more balanced. If your data consists of:

One thousand records with the date 2009-01-01
One thousand records with the date 2009-01-02
…
One thousand records with the date 2009-12-31

The cardinality or uniqueness of that column is low given the fact that you’ll have 365000 rows with blocks of one thousand having the same key. If you consider 125 different IP addresses per day generating those same thousand records, the cardinality or uniqueness of the IP addresses will be very high.

In order to show the performance differences in multiple indexing schemes and representations, a table has been created with an Unsigned Int column for the IP address, a varchar(15) for the IP address, a date column, and a varchar(80) for some text data. Because of the way the MySQL query processor works, it is possible to construct your query so that the results are answered from the index and the data file is never hit. A test sample was created that will be used for all of the tests. The file will be indexed, optimized, and the test run five times with the cumulative time used. The sample data that generates the queries against the database include 48000 of the ten million rows, plus 2000 randomly generated queries. Those results are then shuffled and written to a file for the tests. Testing hits versus misses emulates real world situations a little more accurately. All of the code used to run these tests is included in this post.

Test Setup

Creation of the table:

CREATE TABLE `querytest` (
  `iip` int(10) unsigned DEFAULT NULL,
  `ipv` varchar(15) DEFAULT NULL,
  `date` date DEFAULT NULL,
  `randomtext` text
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

Filling the table with data:

#!/usr/bin/python

import MySQLdb
import random
import datetime
import time

lipsum = """
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi gravida congue nisi, nec auctor leo placerat nec. In hac habitasse platea dictumst. In rutrum blandit velit et varius. Integer commodo ipsum ut diam placerat feugiat. Curabitur viverra erat ut felis cursus mollis. Sed tempus tempor faucibus. Etiam eget arcu massa, eget dictum sapien. Nullam euismod purus vitae risus ultrices tempus. Mauris semper rhoncus lectus, sit amet laoreet mauris tincidunt et. Duis ut mauris massa. Nam semper, enim id fermentum tristique, ligula velit suscipit lacus, vitae ultrices mi arcu sit amet felis. Ut sit amet tellus eget lorem gravida malesuada.

Integer nec massa quis mauris porta laoreet. Curabitur tincidunt nunc at mauris porttitor auctor. Mauris auctor faucibus tortor dignissim sodales. Sed ut tellus nisi, laoreet malesuada tortor. Vivamus blandit neque et nunc fringilla quis dignissim felis tincidunt. Nam nec varius orci. Duis pretium magna id urna fermentum commodo. Aliquam sollicitudin imperdiet leo eget ullamcorper. Quisque id mauris nec purus pulvinar bibendum. Fusce nunc metus, viverra in iaculis id, tempus nec neque. Aenean ac diam arcu, vitae condimentum lectus. Vivamus cursus iaculis tortor eget bibendum. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Aenean elementum odio et nisl ornare at sodales eros porta. Duis mollis tincidunt neque, sed pulvinar enim ultrices a. Sed laoreet nunc ut nisl luctus a egestas quam luctus. Pellentesque non dui et neque ullamcorper condimentum ac ut turpis. Etiam a lectus odio, vitae bibendum arcu. Nulla egestas dolor ligula.

Quisque rhoncus neque ultrices mi lacinia tempus. Sed scelerisque libero dui, quis vulputate leo. Phasellus nibh ante, viverra sed cursus ac, dictum et lectus. Suspendisse potenti. Ut dapibus augue vitae sem convallis in iaculis nibh bibendum. Mauris eu sapien in lacus pharetra fermentum. Etiam eleifend vulputate velit, a tempor augue ultrices vitae. Vestibulum varius orci ac justo adipiscing quis dignissim odio porttitor. Nam ac metus leo. Ut a porttitor lectus. Nunc accumsan ante non eros feugiat suscipit.

Nulla facilisi. Nam molestie dignissim purus sed lacinia. Etiam tristique, eros vel condimentum fermentum, ipsum justo vulputate erat, sed faucibus nunc nisl id tellus. Aliquam a tempus leo. Nullam et sem nunc. Suspendisse potenti. Quisque ante lorem, aliquam sed aliquet vel, malesuada sit amet nisl. Vestibulum tristique velit pellentesque sapien ultrices non gravida ante blandit. Donec luctus nunc dictum felis feugiat sollicitudin. Nam lectus mi, porttitor sed adipiscing ac, pharetra a orci. Ut vitae eros vitae metus. 
"""

db = MySQLdb.connect(host="localhost", user="querytest", passwd="qt1qt1", db="querytest")
cursor = db.cursor()

length = len(lipsum)
jan_1_2009 = time.mktime((2009, 1, 1, 0, 0, 0, 0, 0, 0))

for i in range (1, 10000001):
  
  # generate a random IP address
  rand_ip = random.randint(1,4294967295)

  # pull a random piece of text from lipsum with a random length
  start_pos = random.randint(1,length)
  end_pos = start_pos + random.randint(200,2000)
  random_text = lipsum[start_pos:end_pos]

  # pick a random date in 2009
  rand_date = time.strftime("%Y-%m-%d",time.gmtime(jan_1_2009 + random.randint(1,365*60*60*24)))

  cursor.execute("insert into querytest (iip,ipv,date,randomtext) values (%s,inet_ntoa(%s),%s,%s)", (rand_ip, rand_ip, rand_date, random_text))
  
cursor.close ()
db.close ()

Generate test set:

#!/usr/bin/python

import MySQLdb
import random
import datetime
import time
import socket
import struct

db = MySQLdb.connect(host="localhost", user="querytest", passwd="qt1qt1", db="querytest")
cursor = db.cursor()

jan_1_2009 = time.mktime((2009, 1, 1, 0, 0, 0, 0, 0, 0))

cursor.execute("select iip,ipv,date from querytest order by rand() limit 48000")

data = list(cursor.fetchall())

for i in range (1, 2001):
  
  # generate a random IP address
  rand_ip = random.randint(1,4294967295)

  # pick a random date in 2009
  rand_date = time.strftime("%Y-%m-%d",time.gmtime(jan_1_2009 + random.randint(1,365*60*60*24)))

  data.append((rand_ip, socket.inet_ntoa(struct.pack('L',rand_ip)), rand_date))

random.shuffle(data)
for datum in data:
  print "%s,%s,%s" % (datum[0], datum[1], datum[2])

cursor.close ()
db.close ()

At this point we have created the table, filled it with ten million rows, and generated a fifty thousand row query set to run against the table. Now, we need to categorize the theories to see whether cardinality plays as large a role as it used to.

The following tests will be performed

Index of iip,date

* Use the unsigned int representation of the IP address and the date
* Use the text representation of the IP address passed through inet_aton() and the date

Index of ipv, date

* Use the text representation of the IP address and the date
* Use the unsigned int representation of the IP address passed through inet_ntoa() and the date

Index of date,iip

* Use date and the unsigned int representation of the IP address
* Use date and the text representation of the IP address passed through inet_aton()

Index of date,ipv

* Use date and the unsigned int representation of the IP address
* Use date and the text representation of the IP address passed through inet_aton()

Each of the above tests will be run twice, once with select * and once with select ipv,date.

Benchmark Code

#!/usr/bin/python

import MySQLdb
import random
import datetime
import time
import socket
import struct
import array

def run_query(query, data, columna, columnb):
    for datum in data:
      cursor.execute(query, (datum[columna], datum[columnb]))
      result = cursor.fetchall()

query_tests = [
               ['create index querytest on querytest (iip,date)', 
                'select * from querytest where iip=%s and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (iip,date) using HASH', 
                'select * from querytest where iip=%s and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (iip,date)', 
                'select iip,date from querytest where iip=%s and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (iip,date) using HASH', 
                'select iip,date from querytest where iip=%s and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (iip,date)', 
                'select * from querytest where iip=inet_aton(%s) and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (iip,date) using HASH', 
                'select * from querytest where iip=inet_aton(%s) and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (iip,date)', 
                'select iip,date from querytest where iip=inet_aton(%s) and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (iip,date) using HASH', 
                'select iip,date from querytest where iip=inet_aton(%s) and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (ipv,date)', 
                'select * from querytest where ipv=%s and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (ipv,date) using HASH', 
                'select * from querytest where ipv=%s and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (ipv,date)', 
                'select ipv,date from querytest where ipv=%s and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (ipv,date) using HASH', 
                'select ipv,date from querytest where ipv=%s and date=%s',
                1,
                2
               ],
               ['create index querytest on querytest (ipv,date)', 
                'select * from querytest where ipv=inet_ntoa(%s) and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (ipv,date) using HASH', 
                'select * from querytest where ipv=inet_ntoa(%s) and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (ipv,date)', 
                'select ipv,date from querytest where ipv=inet_ntoa(%s) and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (ipv,date) using HASH', 
                'select ipv,date from querytest where ipv=inet_ntoa(%s) and date=%s',
                0,
                2
               ],
               ['create index querytest on querytest (date,iip)', 
                'select * from querytest where date=%s and iip=%s',
                2,
                0
               ],
               ['create index querytest on querytest (date,iip) using HASH', 
                'select * from querytest where date=%s and iip=%s',
                2,
                0
               ],
               ['create index querytest on querytest (date,iip)', 
                'select iip,date from querytest where date=%s and iip=%s',
                2,
                0
               ],
               ['create index querytest on querytest (date,iip) using HASH', 
                'select iip,date from querytest where date=%s and iip=%s',
                2,
                0
               ],
               ['create index querytest on querytest (date,iip)', 
                'select * from querytest where date=%s and iip=inet_aton(%s)',
                2,
                1
               ],
               ['create index querytest on querytest (date,iip) using HASH', 
                'select * from querytest where date=%s and iip=inet_aton(%s)',
                2,
                1
               ],
               ['create index querytest on querytest (date,iip)', 
                'select iip,date from querytest where date=%s and iip=inet_aton(%s)',
                2,
                1
               ],
               ['create index querytest on querytest (date,iip) using HASH', 
                'select iip,date from querytest where date=%s and iip=inet_aton(%s)',
                2,
                1
               ],
               ['create index querytest on querytest (date,ipv)', 
                'select * from querytest where date=%s and ipv=%s',
                2,
                1
               ],
               ['create index querytest on querytest (date,ipv) using HASH', 
                'select * from querytest where date=%s and ipv=%s',
                2,
                1
               ],
               ['create index querytest on querytest (date,ipv)', 
                'select ipv,date from querytest where date=%s and ipv=%s',
                2,
                1
               ],
               ['create index querytest on querytest (date,ipv) using HASH', 
                'select ipv,date from querytest where date=%s and ipv=%s',
                2,
                1
               ],
               ['create index querytest on querytest (date,ipv)', 
                'select * from querytest where date=%s and ipv=inet_ntoa(%s)',
                2,
                0
               ],
               ['create index querytest on querytest (date,ipv) using HASH', 
                'select * from querytest where date=%s and ipv=inet_ntoa(%s)',
                2,
                0
               ],
               ['create index querytest on querytest (date,ipv)', 
                'select ipv,date from querytest where date=%s and ipv=inet_ntoa(%s)',
                2,
                0
               ],
               ['create index querytest on querytest (date,ipv) using HASH', 
                'select ipv,date from querytest where date=%s and ipv=inet_ntoa(%s)',
                2,
                0
               ],
              ]

db = MySQLdb.connect(host="localhost", user="querytest", passwd="qt1qt1", db="querytest")
cursor = db.cursor()

queries = open('testquery.txt').readlines()

query_array = []
for query_data in queries:
  query_array.append(query_data.rstrip('\n').split(','))


for test in query_tests:
  try:
    cursor.execute('alter table querytest drop index querytest')
  except:
    pass
  cursor.execute(test[0])
  cursor.execute('optimize table querytest')

  print "Test: %s\n with Index: %s" % (test[1], test[0])
  start_time = time.time()

  for loop in range (1,6):
    run_query(test[1], query_array, test[2], test[3])

  end_time = time.time()
  print "Duration: %f seconds\n" % (end_time - start_time)

cursor.close ()
db.close ()

Miscellaneous notes

P4/3.0ghz, 2gb RAM, Debian 3/Squeeze, Linux 2.6.31.1, WD 7200RPM SATA drive, SuperMicro P4SCI Motherboard

There are multiple tests that could have been run without dropping the index, recreating the index and optimizing the table. When testing a more limited set, results were a little sporadic due to a smaller initial test set and portions of the table and index being cached in the kernel cache. To ensure more consistent test results, every test was run in a consistent manner.

Benchmark Results

Test: select * from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 679.169198 seconds

Test: select * from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date) using HASH
Duration: 692.634291 seconds

Test: select iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 179.039791 seconds

Test: select iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date) using HASH
Duration: 178.993962 seconds

Test: select * from querytest where iip=inet_aton(%s) and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 672.836734 seconds

Test: select * from querytest where iip=inet_aton(%s) and date=%s
 with Index: create index querytest on querytest (iip,date) using HASH
Duration: 606.268787 seconds

Test: select iip,date from querytest where iip=inet_aton(%s) and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 195.253512 seconds

Test: select iip,date from querytest where iip=inet_aton(%s) and date=%s
 with Index: create index querytest on querytest (iip,date) using HASH
Duration: 195.222058 seconds

Test: select * from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date)
Duration: 741.876227 seconds

Test: select * from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date) using HASH
Duration: 639.109309 seconds

Test: select ipv,date from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date)
Duration: 167.049333 seconds

Test: select ipv,date from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date) using HASH
Duration: 167.016152 seconds

Test: select * from querytest where ipv=inet_ntoa(%s) and date=%s
 with Index: create index querytest on querytest (ipv,date)
Duration: 578.565762 seconds

Test: select * from querytest where ipv=inet_ntoa(%s) and date=%s
 with Index: create index querytest on querytest (ipv,date) using HASH
Duration: 655.869390 seconds

Test: select ipv,date from querytest where ipv=inet_ntoa(%s) and date=%s
 with Index: create index querytest on querytest (ipv,date)
Duration: 181.555567 seconds

Test: select ipv,date from querytest where ipv=inet_ntoa(%s) and date=%s
 with Index: create index querytest on querytest (ipv,date) using HASH
Duration: 181.230911 seconds

Test: select * from querytest where date=%s and iip=%s
 with Index: create index querytest on querytest (date,iip)
Duration: 655.928799 seconds

Test: select * from querytest where date=%s and iip=%s
 with Index: create index querytest on querytest (date,iip) using HASH
Duration: 637.146124 seconds

Test: select iip,date from querytest where date=%s and iip=%s
 with Index: create index querytest on querytest (date,iip)
Duration: 181.637912 seconds

Test: select iip,date from querytest where date=%s and iip=%s
 with Index: create index querytest on querytest (date,iip) using HASH
Duration: 181.512190 seconds

Test: select * from querytest where date=%s and iip=inet_aton(%s)
 with Index: create index querytest on querytest (date,iip)
Duration: 603.553238 seconds

Test: select * from querytest where date=%s and iip=inet_aton(%s)
 with Index: create index querytest on querytest (date,iip) using HASH
Duration: 605.363284 seconds

Test: select iip,date from querytest where date=%s and iip=inet_aton(%s)
 with Index: create index querytest on querytest (date,iip)
Duration: 196.680399 seconds

Test: select iip,date from querytest where date=%s and iip=inet_aton(%s)
 with Index: create index querytest on querytest (date,iip) using HASH
Duration: 194.746056 seconds

Test: select * from querytest where date=%s and ipv=%s
 with Index: create index querytest on querytest (date,ipv)
Duration: 657.619028 seconds

Test: select * from querytest where date=%s and ipv=%s
 with Index: create index querytest on querytest (date,ipv) using HASH
Duration: 686.560066 seconds

Test: select ipv,date from querytest where date=%s and ipv=%s
 with Index: create index querytest on querytest (date,ipv)
Duration: 172.222691 seconds

Test: select ipv,date from querytest where date=%s and ipv=%s
 with Index: create index querytest on querytest (date,ipv) using HASH
Duration: 172.079220 seconds

Test: select * from querytest where date=%s and ipv=inet_ntoa(%s)
 with Index: create index querytest on querytest (date,ipv)
Duration: 726.031732 seconds

Test: select * from querytest where date=%s and ipv=inet_ntoa(%s)
 with Index: create index querytest on querytest (date,ipv) using HASH
Duration: 678.099808 seconds

Test: select ipv,date from querytest where date=%s and ipv=inet_ntoa(%s)
 with Index: create index querytest on querytest (date,ipv)
Duration: 185.415666 seconds

Test: select ipv,date from querytest where date=%s and ipv=inet_ntoa(%s)
 with Index: create index querytest on querytest (date,ipv) using HASH
Duration: 185.280880 seconds

Conclusions

Based on the data, I think we can say that the argument of B-Tree versus Hash doesn’t seem to make much difference. Neither is consistently better, and since the data and query test is identical, the results don’t really point to a clear winner. Avoiding Select * and pulling only the required fields makes a difference and if your result can be answered from the index rather than the data file, there is a substantial boost. Analysis of the results suggests that cardinality isn’t as important as it used to be. I am devising a method to further test cardinality as I do believe that live data will have somewhat different results from data after an optimize table has been run.

The winner in this case is:

Test: select ipv,date from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date)
Duration: 167.049333 seconds

Test: select ipv,date from querytest where ipv=%s and date=%s
 with Index: create index querytest on querytest (ipv,date) using HASH
Duration: 167.016152 seconds

I had actually expected int represented as unsigned int would be the fastest. However, there is probably a reasonable explanation why these two queries are slower:

Test: select iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 179.039791 seconds

Test: select iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date) using HASH
Duration: 178.993962 seconds

Data in MySQL is represented as binary. The IP stored as an unsigned int takes 4 bytes, and the date takes 3. The key length in this case would be 7 bytes versus the index on IP stored as varchar(15) and the date taking 18 bytes. Even though the index in the second case is almost three times the size of the unsigned int IP, the MySQL client library converts all binary data to ASCII when communicating to avoid endian issues. That extra conversion results in a slightly slower result — measurable when you do 250000 queries against a 10 million record database.

A quick modification of the test shows the results of select *, versus select keyvaluea,keyvalueb and select data,keyvalueb. As you can see from the results below, MySQL will answer queries from the index if it doesn’t need to hit the data file.

Test: select * from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 637.420786 seconds

Test: select iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 178.434477 seconds

Test: select ipv,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 690.804990 seconds

Test: select inet_ntoa(iip) as iip,date from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 183.817643 seconds

If you can structure your data well, there are significant performance gains to be had.

What does this mean?

Do you store IPs as unsigned int in the database? If you use varchar(15) or char(15), you’re talking about an eleven or ten byte savings per record at the expense of some conversion time. varchar uses 1 character to store the length of the stored data plus the length of the data. char is a fixed length based on the column length you specify.

Make sure you return only the columns that you need in your calculations — especially if you are running MySQL over a network.

Try to create your index to match the conditions that you are looking for, and, when possible, if you are searching for the result from a particular column, consider adding it to the index as well.

Always use count(*) rather than count(column) unless there is a valid reason for that column to contain NULL.

The Effect of count(*) versus count(date)

count(*) gives you the number of rows in the set that match the criteria you have set. count(date) counts the number of rows in the set that match the criteria where the date is not null. Many times, you’ll see someone do a count(id), and id by definition is a primary key, auto_increment and cannot be null. Because count(column) must read the table to ensure that the column specified is not null, it is forced to check every key, or, read the table for all of the matching rows to make sure the column retrieved doesn’t contain a null value. If the column being counted is one of the keys in the index, the performance change won’t be as dramatic. By counting a column that isn’t in the key and having to read the data, count(column) is considerably slower.

Results when the counted column is within the key and only 1 or 0 rows are expected:

Test: select count(*) as ct from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 175.727338 seconds

Test: select count(iip) as ct from querytest where iip=%s and date=%s
 with Index: create index querytest on querytest (iip,date)
Duration: 176.495198 seconds

When count returns more than one row, you can see the effect is much more detrimental. The first iteration of this test took so long that I shortened it to do five iterations of 100 queries. After 4 hours, and 18% complete, I shortened the test to do one iteration of ten queries. The results clearly demonstrate the issue without taking 20+ hours to run a single simple benchmark. Simply stated, unless you really have a valid reason to check your results to see if the column is null, DON’T!

Test: select count(*) as ct from querytest where date=%s
 with Index: create index querytest on querytest (date,iip)
Duration: 0.408268 seconds

Test: select count(ipv) as ct from querytest where date=%s
 with Index: create index querytest on querytest (date,iip)
Duration: 3085.770998 seconds

The Fine Print

* Index columns used in your where conditions
* B-Tree versus Hash doesn’t appear to materially affect results
* storing IP as char(15) if the data is being returned to the client can be faster than storing an IP as an unsigned int. If the IP is not fetched but only used in comparisons, unsigned int is probably the better choice.
* Consider adding that extra column to your index to prevent MySQL from having to read the data file. Answering your query from the index is significantly faster.
* count(*) rather than count(column)

Live data will not act precisely as the benchmark — what live scenario ever does? But, I believe the tests above should show some of the performance gains available by structuring your tables and queries.

While MySQL 4, 5.0 and 5.1 will reorder conditions to match the index key, there are some significant performance gains from 4.x to 5.0. MySQL 5.1 didn’t show considerable gains from MySQL 5.0, but, there are some minor speed increases.

I heard a comment from a developer the other day:

You don’t need indexes on small tables.

So I asked what the definition of a small table was. He said, anything with a few hundred rows. So I said, 2300 rows? Well….. 24000 rows? Well….. 292000 rows? That’s large. I showed him unindexed queries in his application dealing with tables that had 2300, 24000 and 292000 rows.

Avoid tablescans

When MySQL deals with a query that is unindexed, it does a full tablescan to see if each record in the table meets the criteria specified. On a small table, if the query is executed frequently, the MySQL query cache might be able to serve the query. However, on a larger table, or a table with large rows, it must read every row, check the fields, possibly create a temporary table in ram or disk, and return the results. On a small site, you might not notice it, but, on a large system, forcing tablescans on tables with even a few thousand rows will slow things down considerably:

Uptime: 60016 Threads: 11 Questions: 105460332 Slow queries: 197769 Opens: 5819 Flush tables: 1 Open tables: 1320 Queries per second avg: 1757.204

Slow queries are sometimes unavoidable, but, often, slow queries are missing an index.

Use the slow-query log to find potential issues

When analyzing a system to find problems, putting:

log-queries-not-using-indexes

in the my.cnf file and restarting mysql will log the unindexed queries to the slowquery log.

What can be indexed?

The rule of thumb when writing indexes is to write your query in such a way that you reduce the result set as quickly as possible, with the highest cardinality possible. What does this mean?

If you are collecting data of the IP address and the Date, your query against date,ip will actually be worse than ip,date. Imagine receiving 40000 hits to your site on the same date. If you were looking for the number of hits that a particular IP had, you would search the 41 hits they have made over time, and then the 8 that they had today. If you queried by date,ip, you would search 40000 rows then would receive the 8 they had today. Each index you have, adds extra overhead and an index file should be as small as possible. IP addresses can be represented in an unsigned int which takes much less space than the varchar(15) usually used. Remember when you index a varchar field, indexing will spacepad the key to the full length. If you have a variable length field you want indexed, you might be able to figure out the significant portion of that field by finding the average length and adding a few characters for good measure and indexing fieldname(15) rather than the entire field. If a query is longer than the 15 characters, you have still created a significant reduction in the number of rows that it must check.

Cardinality refers to the uniqueness of the data. The more unique the data, the lower the chance that you’ll have thousands of records that match the first criteria. When the data is very similar, the index as built on disk will become imbalanced resulting in slower queries. Since MyISAM and InnoDB use a B-Tree index (or R-Tree if you use a spatial index), data that is similar when inserted, can create a very imbalanced tree which leads to slower lookups. An optimize table can resort and reindex the table to eliminate this, but, you can’t do that on an extremely large, active table without impacting response times.

# Query_time: 0 Lock_time: 0 Rows_sent: 1 Rows_examined: 3323
SELECT * FROM websites_geo where (zoneid = ‘5135’) LIMIT 1;

In this case, zoneid is not indexed on the table websites_geo. Adding an index on zoneid eliminates the tablescan on this query.

Check for equality, not inequality.

An index can only check equality. A query checking to see if values are not equal, cannot be indexed.

# Query_time: 0 Lock_time: 0 Rows_sent: 5 Rows_examined: 2548
SELECT * FROM websites where (id = ‘1056692’ && status != ‘d’ && status != ‘n’) order by rand() LIMIT 5;

# Query_time: 0 Lock_time: 0 Rows_sent: 10 Rows_examined: 2544
SELECT * FROM websites where (status != ‘n’ && status != ‘d’ && traffic > 3000) order by added desc LIMIT 10;

These two queries show two different issues, but, deal with the same fundamental issue. First, id is not indexed which would have at least limited the result set to 9 records rather than 2548. The status check isn’t able to use an index. On the second query, status is checked followed by traffic. There are other queries issued that check status,traffic,clicks_high. When we look at status (which should be an enum or char(1) rather than varchar(1)), we find that there are only 4 values used. By indexing on id,status and status,traffic,clicks_high, we could alter the queries as such:

SELECT * FROM websites where (id = ‘1056692’ && status in (‘g’,’ ‘)) order by rand() LIMIT 5;

SELECT * FROM websites where (status in (‘g’,’ ‘) && traffic > 3000) order by added desc LIMIT 10;

which would result in both queries using an index.

Choose your data types intelligently.

As a secondary point, id (though it is numeric) happens to be a text field. If you index id in this case, you would have to specify a key length.

mysql> select max(length(id)) from websites;
+—————–+
| max(length(id)) |
+—————–+
| 22 |
+—————–+
1 row in set (0.02 sec)

mysql> select avg(length(id)) from websites;
+—————–+
| avg(length(id)) |
+—————–+
| 8.3315 |
+—————–+
1 row in set (0.00 sec)

mysql>

Based on this, we might decide to set the key length to 22 as it is a relatively small number and allows room to grow. Personally, I would have opted to have the id be an unsigned int which would be much smaller, but, the application developer uses alphanumeric id’s which are exposed externally. With sharding, you could use the id throughout the various tables, or, you could map the text id to a numeric id internally for all of the various tables.

There are a number of possible solutions to help any SQL engine perform better. And your data set will dictate some of the things that you can do to make data access quicker.

Helping MySQL Help You

If you do select * from table where condition_a=1 and condition_b=2 in one place, and select * from table where condition_b=2 and condition_a=1, setting up a single index on condition_a,condition_b and adjusting your second query, reversing the conditions to the same order as the keys on the index will increase performance.

Limit your results

Another thing that will help considerably is using a limit clause. So many times a programmer will do: select * from table where condition_a=1 which returns 2300 rows but only the first few rows are used. A limit clause will prevent a lot of data from being fetched by MySQL and buffered waiting for the response. select * from table where condition_a=1 limit 20 would hand you the first 20 records.

Avoid reading the data file, do all your work from the Index

Additionally, if you have a table and only need three of the columns from the result, select fielda,fieldb,fieldc from table where condition_a=1 will return only the three fields. As an added boost, if the fields you are checking can be answered from the index, the query will never hit the actual data file and will be answered from the index. Many times I’ve added a field that wasn’t needed in the index, just to eliminate the lookup of the key in the index then the corresponding read of the data file.

Let MySQL do the work

MySQL reads tables, filters results, can do some calculations. Going through 40000 records to pick the best 100 is still faster in MySQL than allowing PHP to fetch 40000 rows and do calculations and sorts to come up with that 100 rows. Index, optimize, and allow MySQL to do the database work.

Summary

Making MySQL work more efficiently goes a long way towards making your database driven site work better. Adding six indexes to the system resulted in quicker response times and an increase in the transactions per second.

Uptime: 32405 Threads: 1 Questions: 58729705 Slow queries: 64122 Opens: 2911 Flush tables: 1 Open tables: 295 Queries per second avg: 1812.366

Previously, MySQL was generating 3.26 slow queries per second. Now we’re just beneath 2 slow queries per second and our system is processing 55 more transactions per second. There is still a bit more analysis to do to identify the slow queries that are still running and to alter the queries to reverse the inequality checks, but, even just adding indexes to a few tables has helped noticeably. Once the developer is able to make some changes to the application, I’m sure we’ll see an additional speedup.