[Django] #12165: Ability to use SQL functions in queries

[noreply-71]

#12165: Ability to use SQL functions in queries
------------------------------------------+---------------------------------
 Reporter:  premalshah                    |       Owner:  nobody    
   Status:  new                           |   Milestone:  1.2      
Component:  Database layer (models, ORM)  |     Version:  1.0      
 Keywords:  mysql functions               |       Stage:  Unreviewed
Has_patch:  0                             |  
------------------------------------------+---------------------------------
 MySQL has text data type fields like TINYTEXT, TEXT, MEDIUMTEXT, and
 LONGTEXT and binary data type fields like TINYBLOB, BLOB, MEDIUMBLOB, and
 LONGBLOB. Text fields have a character set, while binary fields dont. We
 store a lot of data in text fields. Recently, during a database audit, it
 was recommended by MySQL experts to compress the data we store in the text
 fields and move it into the binary fields since text fields cannot store
 compressed binary data. There are 2 ways of doing this.

  1. Create a CompressedTextField custom model field. This code works fine.
 There are other side effects though.
   * MySQL query log now has binary characters. So, if you tailiing or
 catting it, its not a good experience.
   * You are compressing/uncompressing on the web server. Most people have
 powerful database servers and less powerful web servers. So, it can hurt
 performance.

 The code is as under.
 {{{
 import zlib

 class CompressedField(Field):
     __metaclass__ = SubfieldBase

     def to_python(self, value):
         try:
             value = zlib.decompress(value[4:])
         except (TypeError, zlib.error):
             return value

     def get_db_prep_value(self, value):
         if not value: return None
         c = zlib.compress(value)
         return buffer(struct.pack('I', len(value)) + c)

     def get_internal_type(self):
         return 'TextField'

     def db_type(self):
         return 'longblob'
 }}}

  2. MySQL has compress and uncompress functions.
 http://dev.mysql.com/doc/refman/5.1/en/encryption-functions.html
 {{{
 COMPRESS(string_to_compress)
 ex: INSERT INTO <table_name> (col1) values
 (COMPRESS(<string_to_compress>));
 UNCOMPRESS(string_to_uncompress)
 ex: SELECT UNCOMPRESS(<string_to_uncompress>) FROM <table_name>;
 }}}

 We can create a custom model field as under.
 {{{
 class BlobField(Field):
     __metaclass__ = SubfieldBase

     def get_internal_type(self):
         return 'TextField'

     def db_type(self):
         return 'longblob'
 }}}

 Then a field can be defined as
 {{{
 class Test(models.Model):
     blog = models.BlobField(input_function='compress',
 output_function='uncompress')
 }}}

 compress and uncompress are the mysql functions are wrapped around the
 text or the column name when django constructs the query. This is just one
 example of database functions that could be used by developers via the
 Django ORM. Someone might find use for Encrypt and Decrypt or Encode and
 Decode.

 Any thoughts?

--
Ticket URL: <http://code.djangoproject.com/ticket/12165>
Django <http://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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[noreply-71]

#12165: Ability to use SQL functions in queries
---------------------------------------------------+------------------------
          Reporter:  premalshah                    |         Owner:  nobody        
            Status:  new                           |     Milestone:  1.2            
         Component:  Database layer (models, ORM)  |       Version:  1.0            
        Resolution:                                |      Keywords:  mysql functions
             Stage:  Design decision needed        |     Has_patch:  0              
        Needs_docs:  0                             |   Needs_tests:  0              
Needs_better_patch:  0                             |  
---------------------------------------------------+------------------------
Changes (by tobias):

  * needs_better_patch:  => 0
  * stage:  Unreviewed => Design decision needed
  * needs_tests:  => 0
  * needs_docs:  => 0

Comment:

 Not sure if it is possible, but have you tried writing a Field that calls
 compress/uncompress on the database side?  It may require diving deeper
 into the Django code.  When doing this you should also think about how it
 will work for other database backends.  Do postgres, sqlite, and oracle
 have similar methods?

 As an alternative, you could probably accomplish what you need with the
 Model.objects.raw() being developed in #11863.

--
Ticket URL: <http://code.djangoproject.com/ticket/12165#comment:1>
Django <http://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

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[noreply-71]

#12165: Ability to use SQL functions in queries
---------------------------------------------------+------------------------
          Reporter:  premalshah                    |         Owner:  nobody        
            Status:  new                           |     Milestone:  1.2            
         Component:  Database layer (models, ORM)  |       Version:  1.0            
        Resolution:                                |      Keywords:  mysql functions
             Stage:  Design decision needed        |     Has_patch:  0              
        Needs_docs:  0                             |   Needs_tests:  0              
Needs_better_patch:  0                             |  
---------------------------------------------------+------------------------
Comment (by premalshah):

 We are using MySQL which does not support user defined data types unlike
 postgres. In postgres, you could define a user defined field and tell it
 to use postgress functions when data is saved/retrieved from the database.
 This is why the responsibility of compress/decompress falls on the query
 or the application. There are other applications too. The mysql datetime
 field uses 8 bytes while the timestamp field (which stored the time as a
 unix timestamp) uses 4 bytes. If you have a few million rows, there is
 definitely some savings. mysql has functions like FROM_TIMESTAMP and
 UNIX_TIMESTAMP which convert the timestamps back and forth. It would be
 powerful to use these functions in the queries while still  operating on
 datetime objects in the python application. Any thoughts?

--
Ticket URL: <http://code.djangoproject.com/ticket/12165#comment:2>
Django <http://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

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