old_contents: #coding: utf-8
  
  from django.db import models
  
  class Substance(models.Model):
      name = models.TextField(unique=True, db_index=True)
      alias = models.TextField()
  
  class Station(models.Model):
      name = models.TextField(unique=True, db_index=True)
      alias = models.TextField()
  
  class Unit(models.Model):
      name = models.TextField(unique=True, db_index=True)
  
  class Measurement(models.Model):
      station = models.ForeignKey(Station)
      substance = models.ForeignKey(Substance)
      unit = models.ForeignKey(Unit)
      value = models.FloatField()
      performed = models.DateTimeField()
      written = models.DateTimeField(auto_now=True)
  
new_contents: #coding: utf-8
  
  from django.db import models
  
  class Substance(models.Model):
      name = models.TextField(unique=True, db_index=True)
      alias = models.TextField()
  
  class Station(models.Model):
      name = models.TextField(unique=True, db_index=True)
      alias = models.TextField()
  
  class Unit(models.Model):
      name = models.TextField(unique=True, db_index=True)
  
  class Measurement(models.Model):
      station = models.ForeignKey(Station)
      substance = models.ForeignKey(Substance)
      unit = models.ForeignKey(Unit)
      value = models.FloatField()
      performed = models.DateTimeField()
      written = models.DateTimeField(auto_now=True)
  
      class Meta:
          index_together = [
              ['station', 'substance', 'unit', 'performed']
          ]
  
subject: Add index for Measurement model