工作原理：

1、新建一张一模一样的表，表名一般是_new后缀
2、在这个新表执行更改字段操作
3、原表上加三个触发器，DELETE/UPDATE/INSERT，将原表中要执行的语句也在新表中执行
4、删除旧表以及触发器：
    DROP TABLE IF EXISTS `_xx_old`
    DROP TRIGGER IF EXISTS `pt_osc_xx_xx_del`;
    DROP TRIGGER IF EXISTS `pt_osc_xx_xx_upd`;
    DROP TRIGGER IF EXISTS `pt_osc_xx_xx_ins`;

使用方法：

pt-online-schema-change [OPTIONS] DSN

Options:

  --alter=s                        The schema modification, without the ALTER   
                                   TABLE keywords
  --alter-foreign-keys-method=s    How to modify foreign keys so they reference 
                                   the new table
  --[no]analyze-before-swap        Execute ANALYZE TABLE on the new table       
                                   before swapping with the old one (default    
                                   yes)
  --ask-pass                       Prompt for a password when connecting to     
                                   MySQL
  --charset=s                  -A  Default character set
  --[no]check-alter                Parses the --alter specified and tries to    
                                   warn of possible unintended behavior (       
                                   default yes)
  --check-interval=m               Sleep time between checks for --max-lag (    
                                   default 1).  Optional suffix s=seconds, m=   
                                   minutes, h=hours, d=days; if no suffix, s is 
                                   used.
  --[no]check-plan                 Check query execution plans for safety (     
                                   default yes)
  --[no]check-replication-filters  Abort if any replication filter is set on    
                                   any server (default yes)
  --check-slave-lag=s              Pause the data copy until this replica's lag 
                                   is less than --max-lag
  --[no]check-unique-key-change    Avoid pt-online-schema-change to run if the  
                                   specified statement for --alter is trying to 
                                   add an unique index (default yes)
  --chunk-index=s                  Prefer this index for chunking tables        
  --chunk-index-columns=i          Use only this many left-most columns of a -- 
                                   chunk-index
  --chunk-size=z                   Number of rows to select for each chunk
                                   copied (default 1000)
  --chunk-size-limit=f             Do not copy chunks this much larger than the
                                   desired chunk size (default 4.0)
  --chunk-time=f                   Adjust the chunk size dynamically so each
                                   data-copy query takes this long to execute (
                                   default 0.5)
  --config=A                       Read this comma-separated list of config
                                   files; if specified, this must be the first
                                   option on the command line
  --critical-load=A                Examine SHOW GLOBAL STATUS after every
                                   chunk, and abort if the load is too high (
                                   default Threads_running=50)
  --data-dir=s                     Create the new table on a different
                                   partition using the DATA DIRECTORY feature
  --database=s                 -D  Connect to this database
  --default-engine                 Remove ENGINE from the new table
  --defaults-file=s            -F  Only read mysql options from the given file
  --[no]drop-new-table             Drop the new table if copying the original
                                   table fails (default yes)
  --[no]drop-old-table             Drop the original table after renaming it (
                                   default yes)
  --[no]drop-triggers              Drop triggers on the old table. --no-drop-
                                   triggers forces --no-drop-old-table (default
                                   yes)
  --dry-run                        Create and alter the new table, but do not
                                   create triggers, copy data, or replace the
                                   original table
  --execute                        Indicate that you have read the
                                   documentation and want to alter the table
  --force                          This options bypasses confirmation in case
                                   of using alter-foreign-keys-method = none ,
                                   which might break foreign key constraints
  --force-concat-enums             The NibbleIterator in Percona Toolkit can
                                   detect indexes having ENUM fields and if the
                                   items it has are sorted or not
  --help                           Show help and exit
  --host=s                     -h  Connect to host
  --max-flow-ctl=f                 Somewhat similar to --max-lag but for PXC
                                   clusters
  --max-lag=m                      Pause the data copy until all replicas' lag
                                   is less than this value (default 1s).
                                   Optional suffix s=seconds, m=minutes, h=
                                   hours, d=days; if no suffix, s is used.
  --max-load=A                     Examine SHOW GLOBAL STATUS after every
                                   chunk, and pause if any status variables are
                                   higher than their thresholds (default
                                   Threads_running=25)
  --new-table-name=s               New table name before it is swapped. %T is
                                   replaced with the original table name (
                                   default %T_new)
  --null-to-not-null               Allows MODIFYing a column that allows NULL
                                   values to one that doesn't allow them
  --only-same-schema-fks           Check foreigns keys only on tables on the
                                   same schema than the original table
  --password=s                 -p  Password to use when connecting
  --pause-file=s                   Execution will be paused while the file
                                   specified by this param exists
  --pid=s                          Create the given PID file
  --plugin=s                       Perl module file that defines a
                                   pt_online_schema_change_plugin class
  --port=i                     -P  Port number to use for connection
  --preserve-triggers              Preserves old triggers when specified
  --print                          Print SQL statements to STDOUT
  --progress=a                     Print progress reports to STDERR while
                                   copying rows (default time,30)
  --quiet                      -q  Do not print messages to STDOUT (disables --
                                   progress)
  --recurse=i                      Number of levels to recurse in the hierarchy
                                   when discovering replicas
  --recursion-method=a             Preferred recursion method for discovering
                                   replicas (default processlist,hosts)
  --remove-data-dir                If the original table was created using the
                                   DATA DIRECTORY feature, remove it and create
                                   the new table in MySQL default directory
                                   without creating a new isl file (default no)
  --set-vars=A                     Set the MySQL variables in this comma-
                                   separated list of variable=value pairs
  --skip-check-slave-lag=d         DSN to skip when checking slave lag
  --slave-password=s               Sets the password to be used to connect to
                                   the slaves
  --slave-user=s                   Sets the user to be used to connect to the
                                   slaves
  --sleep=f                        How long to sleep (in seconds) after copying
                                   each chunk (default 0)
  --socket=s                   -S  Socket file to use for connection
  --statistics                     Print statistics about internal counters
  --[no]swap-tables                Swap the original table and the new, altered
                                   table (default yes)
  --tries=a                        How many times to try critical operations
  --user=s                     -u  User for login if not current user
  --version                        Show version and exit
  --[no]version-check              Check for the latest version of Percona
                                   Toolkit, MySQL, and other programs (default
                                   yes)

Option types: s=string, i=integer, f=float, h/H/a/A=comma-separated list, d=DSN, z=size, m=time

Rules:

  --dry-run and --execute are mutually exclusive.
  This tool accepts additional command-line arguments. Refer to the SYNOPSIS and usage information for details.

DSN syntax is key=value[,key=value...]  Allowable DSN keys:

  KEY  COPY  MEANING
  ===  ====  =============================================
  A    yes   Default character set
  D    no    Database for the old and new table
  F    yes   Only read default options from the given file
  P    yes   Port number to use for connection
  S    yes   Socket file to use for connection
  h    yes   Connect to host
  p    yes   Password to use when connecting
  t    no    Table to alter
  u    yes   User for login if not current user

  If the DSN is a bareword, the word is treated as the 'h' key.

Options and values after processing arguments:

  --alter                          (No value)
  --alter-foreign-keys-method      (No value)
  --analyze-before-swap            TRUE
  --ask-pass                       FALSE
  --charset                        (No value)
  --check-alter                    TRUE
  --check-interval                 1
  --check-plan                     TRUE
  --check-replication-filters      TRUE
  --check-slave-lag                (No value)
  --check-unique-key-change        TRUE
  --chunk-index                    (No value)
  --chunk-index-columns            (No value)
  --chunk-size                     1000
  --chunk-size-limit               4.0
  --chunk-time                     0.5
  --config                         /etc/percona-toolkit/percona-toolkit.conf,/etc/percona-toolkit/pt-online-schema-change.conf,/home/ubuntu/.percona-toolkit.conf,/home/ubuntu/.pt-online-schema-change.conf
  --critical-load                  Threads_running=50
  --data-dir                       (No value)
  --database                       (No value)
  --default-engine                 FALSE
  --defaults-file                  (No value)
  --drop-new-table                 TRUE
  --drop-old-table                 TRUE
  --drop-triggers                  TRUE
  --dry-run                        FALSE
  --execute                        FALSE
  --force                          FALSE
  --force-concat-enums             FALSE
  --help                           TRUE
  --host                           (No value)
  --max-flow-ctl                   (No value)
  --max-lag                        1
  --max-load                       Threads_running=25
  --new-table-name                 %T_new
  --null-to-not-null               FALSE
  --only-same-schema-fks           FALSE
  --password                       (No value)
  --pause-file                     (No value)
  --pid                            (No value)
  --plugin                         (No value)
  --port                           (No value)
  --preserve-triggers              FALSE
  --print                          FALSE
  --progress                       time,30
  --quiet                          FALSE
  --recurse                        (No value)
  --recursion-method               processlist,hosts
  --remove-data-dir                TRUE
  --set-vars
  --skip-check-slave-lag           (No value)
  --slave-password                 (No value)
  --slave-user                     (No value)
  --sleep                          0
  --socket                         (No value)
  --statistics                     FALSE
  --swap-tables                    TRUE
  --tries                          (No value)
  --user                           (No value)
  --version                        FALSE
  --version-check                  FALSE

示例(给数据库test中表test_db增加t3字段，并放到t2后面)：

pt-online-schema-change --user=user --password=password -h 127.0.0.1 --alter "ADD COLUMN t3 int default 0 after t2" P=3306,D=test,t=test_db --no-check-replication-filters --alter-foreign-keys-method=auto --recursion-method=none --print --charset=utf8 --execute