Migrating from MySQL with pgloader

If you want to migrate your data over to PostgreSQL from MySQL then pgloader is the tool of choice!

Most tools around are skipping the main problem with migrating from MySQL, which is to do with the type casting and data sanitizing that needs to be done. pgloader will not leave you alone on those topics.

The Command

To load data with pgloader you need to define in a command the operations in some details. Here's our example for loading the MySQL Sakila Sample Database:

Here's our command:

load database  
     from      mysql://root@localhost/sakila  
     into postgresql:///sakila  
 
 WITH include drop, create tables, no truncate,  
      create indexes, reset sequences, foreign keys  
 
  SET maintenance_work_mem to '128MB', work_mem to '12MB', search_path to 'sakila'  
 
 CAST type datetime to timestamptz  
                drop default drop not null using zero-dates-to-null,  
      type date drop not null drop default using zero-dates-to-null  
 
 MATERIALIZE VIEWS film_list, staff_list  
 
 -- INCLUDING ONLY TABLE NAMES MATCHING ~/film/, 'actor'  
 -- EXCLUDING TABLE NAMES MATCHING ~<ory>  
 
 BEFORE LOAD DO  
 $$ create schema if not exists sakila; $$; 

You can see the full list of options in the pgloader reference manual, with a complete description of the options you see here.

Note that here pgloader will benefit from the meta-data information found in the MySQL database to create a PostgreSQL database capable of hosting the data as described, then load the data.

In particular, some specific casting rules are given here, to cope with date values such as 0000-00-00 that MySQL allows and PostgreSQL rejects for not existing in our calendar. It's possible to add per-column casting rules too, which is useful is some of your tinyint are in fact smallint while some others are in fact boolean values.

Finaly note that we are using the MATERIALIZE VIEWS clause of pgloader: the selected views here will be migrated over to PostgreSQL with their contents.

It's possible to use the MATERIALIZE VIEWS clause and give both the name and the SQL (in MySQL dialect) definition of view, then pgloader creates the view before loading the data, then drops it again at the end.

Loading the data

Let's start the pgloader command with our sakila.load command file:

$ pgloader sakila.load  
... LOG Starting pgloader, log system is ready.  
... LOG Parsing commands from file "/Users/dim/dev/pgloader/test/sakila.load"  
   <WARNING: table "xxx" does not exists have been edited away>  
 
            table name       read   imported     errors            time  
----------------------  ---------  ---------  ---------  --------------  
           before load          1          1          0          0.007s  
       fetch meta data         45         45          0          0.402s  
          create, drop          0         36          0          0.208s  
----------------------  ---------  ---------  ---------  --------------  
                 actor        200        200          0          0.071s  
               address        603        603          0          0.035s  
              category         16         16          0          0.018s  
                  city        600        600          0          0.037s  
               country        109        109          0          0.023s  
              customer        599        599          0          0.073s  
                  film       1000       1000          0          0.135s  
            film_actor       5462       5462          0          0.236s  
         film_category       1000       1000          0          0.070s  
             film_text       1000       1000          0          0.080s  
             inventory       4581       4581          0          0.136s  
              language          6          6          0          0.036s  
               payment      16049      16049          0          0.539s  
                rental      16044      16044          0          0.648s  
                 staff          2          2          0          0.041s  
                 store          2          2          0          0.036s  
             film_list        997        997          0          0.247s  
            staff_list          2          2          0          0.135s  
Index Build Completion          0          0          0          0.000s  
----------------------  ---------  ---------  ---------  --------------  
        Create Indexes         41         41          0          0.964s  
       Reset Sequences          0          1          0          0.035s  
          Foreign Keys         22         22          0          0.254s  
----------------------  ---------  ---------  ---------  --------------  
     Total import time      48272      48272          0          3.502s 

The WARNING messages we see here are expected as the PostgreSQL database is empty when running the command, and pgloader is using the SQL commands DROP TABLE IF EXISTS when the given command uses the include drop option.

Note that the output of the command has been edited to facilitate its browsing online.