The index of the polymerization django

Django (meta-information) metaclass indexed

ORM Query (sql optimization) optimization

Custom aggregate functions

Django metaclass indexed ---- index: a main purpose of the index is to speed up the data retrieval table, the index is the result of an algorithm optimized, so look for the number to be much less. Thus, the index is used to locate.

class Book(models.Model)
    name = models.CharField(max_length=64)

    class Meta:
        # Custom Table Name
        db_table = 'table_name'
        # Joint index: a main purpose of the index is to speed up data retrieval table
        index_together = ('tag1', 'tag2')
        # Joint unique index: two fields considered all repeat repeat
        unique_together = ('tag3', 'tag4')
        # Sort field
        ordering = 'ordering_tag'
        # / Admin table name displayed / in
        verbose_name = 'table_name'

　　

　　

ORM Query (sql optimization) optimized ---- little bit about it

(1) only () to check only certain fields, you want to view other fields, follow-up can point out other fields
    uql1 = Book.objects.all (). only ( "name", "price") # only checked the "name", "price" of two fields
    All fields Book.objects.all () # books are checked again
    print (uq11.first (). gender) # follow-up can point out the other fields, but himself re-execute the query field sql, equivalent to re-checked again

(2) defer () except for certain fields, others are checked
    Book.objects.all (). Defer ( "name", "price") # In addition to "name", "price" of two fields, other fields are checked

(3) values ​​# can not point out the subsequent fields of the other
    Book.objects.values ​​( "name", "price") # only checked the "name", "price" of two fields

　　Custom aggregate functions - can to practice

from django.db.models import Aggregate, CharField

    # Custom name aggregate function
    class Concat (Aggregate): # write a class that inherits Aggregate,
        function = 'GROUP_CONCAT'
        template = '%(function)s(%(distinct)s%(expressions)s)'

        def __init__(self, expression, distinct=False, **extra):
            super(Concat, self).__init__(
                expression,
                distinct='DISTINCT ' if distinct else '',
                output_field=CharField(),
                **extra)

    eg: Book.objects.aggregate( name=Concat("name") )