Phát triển web với PHP và MySQL - p 29 pps

SHOW TABLES [FROM database] Lists tables from the database [LIKE table] currently in use, or from the database called databaseif specified, optionally with table names like table... SHO

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If you leave the database parameter off, the SHOW COLUMNSstatement will default to the

data-base currently in use You can also use the table.columnnotation:

show columns from books.orders;

One other very useful variation of the SHOWstatement can be used to see what privileges a user

has For example, if we run the following, we’ll get the output shown in Figure 11.1:

show grants for bookorama;

F IGURE 11.1

The output of the SHOW GRANTS statement.

The GRANTstatements shown are not necessarily the ones that were executed to give privileges

to a particular user, but rather summary equivalent statements that would produce the user’s

current level of privilege

The SHOW GRANTS statement was added in MySQL version 3.23.4—if you have an ear-lier version, this statement won’t work.

NOTE

There are many other variations of the SHOWstatement A summary of all the variations is

shown in Table 11.6

T ABLE 11.6 SHOW Statement Syntax

SHOW DATABASES Lists available databases, optionally

[LIKE database] with names like database

SHOW TABLES [FROM database] Lists tables from the database

[LIKE table] currently in use, or from the database called

databaseif specified, optionally with table names like table

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SHOW COLUMNS FROM table Lists all the columns in a particular table

[FROM database] [LIKE column] from the database currently in use, or from

the database specified, optionally with col-umn names like column You might use SHOW FIELDSinstead of SHOW COLUMNS

SHOW INDEX FROM table Shows details of all the indexes on a

[FROM database] particular table from the database currently

in use, or from the database called database

if specified You might use SHOW KEYS

instead

SHOW STATUS [LIKE status_item] Gives information about a number of system

items, such as the number of threads run-ning The LIKEclause is used to match against the names of these items, so, for example,‘Thread%’matches the items

‘Threads_cached’,‘Threads_connected’, and ‘Threads_running’

SHOW VARIABLES [LIKE variable_name] Displays the names and values of the

MySQL system variables, such as the ver-sion number The LIKEclause can be used

to match against these in a fashion similar

to SHOW STATUS

SHOW [FULL] PROCESSLIST Displays all the running processes in the

system, that is, the queries that are currently being executed Most users will see their own threads but if they have the PROCESS

privilege, they can see everybody’s processes—including passwords if these are

in queries The queries are truncated to 100 characters by default Using the optional keyword FULLdisplays the full queries

SHOW TABLE STATUS Displays information about each of the

[FROM database] [LIKE database] tables in the database currently being used,

or the database called databaseif it is spec-ified, optionally with a wildcard match This information includes the table type and when each table was last updated

T ABLE 11.6 Continued

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SHOW GRANTS FOR user Shows the GRANTstatements required to give

the user specified in userhis current level

of privilege

Getting Information About Columns with DESCRIBE

As an alternative to the SHOW COLUMNSstatement, you can use the DESCRIBEstatement, similar

to the DESCRIBEstatement in Oracle (another RDBMS) The basic syntax for it is

DESCRIBE table [column];

This will give information about all the columns in the table or a specific column if columnis

specified You can use wildcards in the column name if you like

Understanding How Queries Work with EXPLAIN

The EXPLAINstatement can be used in two ways First, you can use

EXPLAIN table;

This gives very similar output to DESCRIBE tableor SHOW COLUMNS FROM table

The second and more interesting way you can use EXPLAINallows you to see exactly how

MySQL evaluates a SELECTquery To use it this way, just put the word explainin front of a

SELECTstatement

You can use the EXPLAINstatement when you are trying to get a complex query to work and

clearly haven’t got it quite right, or when a query’s taking a lot longer to process than it

should If you are writing a complex query, you can check this in advance by running the

EXPLAINcommand before you actually run the query With the output from this statement, you

can rework your SQL to optimize it if necessary It’s also a handy learning tool

For example, try running the following query on the Book-O-Rama database It produces the

output shown in Figure 11.2

explain

select customers.name

from customers, orders, order_items, books

where customers.customerid = orders.customerid

and orders.orderid = order_items.orderid

and order_items.isbn = books.isbn

and books.title like ‘%Java%’;

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T ABLE 11.6 Continued

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F IGURE 11.2

The output of the EXPLAIN statement.

This might look confusing at first, but it can be very useful Let’s look at the columns in this table one by one

The first column,table, just lists the tables used to answer the query Each row in the result gives more information about how that particular table is used in this query In this case, you can see that the tables used are orders,order_items,customers, and books (We knew this already by looking at the query.)

The typecolumn explains how the table is being used in joins in the query The set of values this column can have is shown in Table 11.7 These values are listed in order from fastest to slowest in terms of query execution It gives you an idea of how many rows need to be read from each table in order to execute a query

T ABLE 11.7 Possible Join Types as Shown in Output from EXPLAIN

constor system The table is read from only once This happens when the table

has exactly one row The type systemis used when it is a system table, and the type constotherwise

eq_ref For every set of rows from the other tables in the join, we read

one row from this table This is used when the join uses all the parts of the index on the table, and the index is UNIQUE or is the primary key

ref For every set of rows from the other tables in the join, we read a

set of rows from this table which all match This is used when the join cannot choose a single row based on the join condition, that is, when only part of the key is used in the join, or if it is not UNIQUE or a primary key

range For every set of rows from the other tables in the join, we read a

set of rows from this table that fall into a particular range

index The entire index is scanned

ALL Every row in the table is scanned

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In the previous example, you can see that one of the tables is joined using eq_ref(books), and

one is joined using ref(order_items), but the other two (ordersand customers) are joined

by using ALL; that is, by looking at every single row in the table

The rowscolumn backs this up—it lists (roughly) the number of rows of each table that has to

be scanned to perform the join You can multiply these together to get the total number of rows

examined when a query is performed We multiply these numbers because a join is like a

prod-uct of rows in different tables—check out Chapter 9, “Working with Your MySQL Database,”

for details Remember that this is the number of rows examined, not the number of rows

returned, and that it is only an estimate—MySQL can’t know the exact number without

per-forming the query

Obviously, the smaller we can make this number, the better At present we have a pretty

negli-gible amount of data in the database, but when the database starts to increase in size, this query

would blow out in execution time We’ll return to this in a minute

The possible_keyscolumn lists, as you might expect, the keys that MySQL might use to join

the table In this case, you can see that the possible keys are all PRIMARYkeys

The keycolumn is either the key from the table MySQL actually used, or NULLif no key was

used You’ll notice that, although there are possible PRIMARYkeys for the ordersand

customerstables, they were not used in this query We’ll look at how to fix this in a minute

The key_lencolumn indicates the length of the key used You can use this to tell whether only

part of a key was used This is relevant when you have keys that consist of more than one

col-umn In this case, where the keys were used (order_itemsand books), the full key was used

The refcolumn shows the columns used with the key to select rows from the table

Finally, the Extracolumn tells you any other information about how the join was performed

The possible values you might see in this column are shown in Table 11.8

T ABLE 11.8 Possible Values for Extra Column as Shown in Output from EXPLAIN

Not exists The query has been optimized to use LEFT JOIN

Range checked for For each row in the set of rows from the other tables in the join,

each record try to find the best index to use, if any

Using filesort Two passes will be required to sort the data (This obviously takes

twice as long.) Using index All information from the table comes from the index—that is, the

rows are not actually looked up

Using temporary A temporary table will need to be created to execute this query

WHEREused A WHEREclause is being used to select rows

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There are several ways you can fix problems you spot in the output from EXPLAIN First, check column types and make sure they are the same This applies particularly to column width Indexes can’t be used to match columns if they have different widths You can fix this

by changing the types of columns to match, or building this in to your design to begin with Second, you can tell the join optimizer to examine key distributions and therefore optimize joins more efficiently using the myisamchkutility You can invoke this by typing

>myisamchk analyze pathtomysqldatabase/table

You can check multiple tables by listing them all on the command line, or by using

>myisamchk analyze pathtomysqldatabase/*.MYI

You can check all tables in all databases by running the following, which will produce the out-put shown in Figure 11.3:

>myisamchk analyze pathtomysqldatadirectory/*/*.MYI

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F IGURE 11.3

This is the output of the EXPLAIN after running myisamchk.

You’ll notice that the way the query is evaluated has changed quite a lot We’re now only

using all the rows in one of the tables (books), which is fine In particular, we’re now using

eq_reffor two of the tables and indexfor the other MySQL is also now using the whole key for order_items(17 characters as opposed to 4 previously)

You’ll also notice the number of rows being used has actually gone up This is probably caused

by the fact that we have little data in the actual database at this point Remember that the num-ber of rows listed is only an estimate—try performing the actual query and checking this If these numbers are way off, the MySQL manual suggests using a straight join and listing the tables in your FROMclause in a different order

Third, you might want to consider adding a new index to the table If this query is a) slow, and b) common, you should seriously consider this If it’s a one-off query that you’ll never use again, such as an obscure report requested once, it won’t be worth the effort, as it will slow other things down We’ll look at how to do this in the next section

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Speeding Up Queries with Indexes

If you are in the situation mentioned previously, in which the possible_keyscolumn from an

EXPLAINcontains some NULLvalues, you might be able to improve the performance of your

query by adding an index to the table in question If the column you are using in your WHERE

clause is suitable for indexing, you can create a new index for it usingALTER TABLElike this:

ALTER TABLE table ADD INDEX (column);

General Optimization Tips

In addition to the previous query optimization tips, there are quite a few things you can do to

generally increase the performance of your MySQL database

Design Optimization

Basically you want everything in your database to be as small as possible You can achieve this

in part with a decent design that minimizes redundancy You can also achieve it by using the

smallest possible data type for columns You should also minimize NULLs wherever possible,

and make your primary key as short as possible

Avoid variable length columns if at all possible (like VARCHAR,TEXT, and BLOB) If your tables

have fixed-length fields they will be faster to use but might take up a little more space

Permissions

In addition to using the suggestions mentioned in the previous section on EXPLAIN, you can

improve the speed of queries by simplifying your permissions We discussed earlier the way

that queries are checked with the permission system before being executed The simpler this

process is, the faster your query will run

Table Optimization

If a table has been in use for a period of time, data can become fragmented as updates and

deletions are processed This will increase the time taken to find things in this table You can

fix this by using the statement

OPTIMIZE TABLE tablename;

or by typing

>myisamchk -r table

at the command prompt

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You can also use the myisamchkutility to sort a table index and the data according to that index, like this:

>myisamchk sort-index sort-records=1 pathtomysqldatadirectory/*/*.MYI

Using Indexes

Use indexes where required to speed up your queries Keep them simple, and don’t create indexes that are not being used by your queries You can check which indexes are being used

by running EXPLAINas shown previously

Use Default Values

Wherever possible, use default values for columns, and only insert data if it differs from the default This reduces the time taken to execute the INSERTstatement

Use Persistent Connections

This particular optimization tip applies particularly to Web databases We’ve already discussed

it elsewhere so this is just a reminder

Other Tips

There are many other minor tweaks you can make to improve performance in particular situa-tions and when you have particular needs The MySQL Web site offers a good set of additional tips You can find it at

http://www.mysql.com

Different Table Types

One last useful thing to discuss before we leave MySQL for the time being is the existence of different types of tables You can choose a table type when you create a table, using

CREATE TABLE table TYPE=type

The possible table types are

• MyISAM This is the default, and what we have used to date This is based on ISAM, which

stands for Indexed Sequential Access Method, a standard method for storing records and

files

• HEAP Tables of this type are stored in memory, and their indexes are hashed This makes

HEAPtables extremely fast, but, in the event of a crash, your data will be lost These char-acteristics make HEAPtables ideal for storing temporary or derived data You should spec-ify the MAX_ROWSin the CREATE TABLEstatement, or these tables can hog all your

memory Also, they cannot have BLOB,TEXT, or AUTO INCREMENTcolumns

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• BDB These tables are transaction safe; that is, they provide COMMITand ROLLBACK capabil-ities They are slower to use than the MyISAMtables, and are based on the Berkeley DB

At the time of writing, these were still being debugged in MySQL version 3.23.21, and will require an extra download in order to be used, available from the MySQL Web site

These additional table types can be useful when you are striving for extra speed or

transac-tional safety

Loading Data from a File

One useful feature of MySQL that we have not yet discussed is the LOAD DATA INFILE

state-ment This can be used to load table data in from a file It executes very quickly

This is a flexible command with many options, but typical usage is something like the

follow-ing:

LOAD DATA INFILE “newbooks.txt” INTO TABLE books;

This will read row data from the file newbooks.txtinto the table books By default, data fields

in the file must be separated by tabs and enclosed in single quotes, and each row must be

sepa-rated by a newline (\n) Special characters must be escaped out with a slash (\) All these

char-acteristics are configurable with the various options of the LOADstatement—see the MySQL

manual for more details

To use the LOAD DATA INFILEstatement, a user must have the FILE privilege discussed earlier

Further Reading

In these chapters on MySQL, we have focused on the uses and parts of the system most

rele-vant to Web development, and to linking MySQL with PHP

If you want to know more, particularly with regard to non-Web applications, or MySQL

administration, you can visit the MySQL Web site at

http://www.mysql.com

You might also want to consult Paul Dubois’ book MySQL, available from New Riders

Publishing

Next

We have now covered the fundamentals of PHP and MySQL In Chapter 12, “Running an

E-commerce Site,” we will look at the e-commerce and security aspects of setting up

database-backed Web sites

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