does django db_index=True index null value?

Question

for example if i have a field name slug = models.CharField(null=True, db_index=True,max_length=50) and while saving data if left slug empty. will database index this saved null value?

This is database-dependent. For PostgreSQL, as far as I remember, it does save it in a bitmap, not in the btree, but yes it is indexed, and thus search should be boosted searching for null values: patshaughnessy.net/2014/11/11/… — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Apr 16, 2020 at 19:00
@WillemVanOnsem you are right for PostgreSQL it does index null from version 8.3 ( 12 years old one) — iklinac
– iklinac, Commented Apr 16, 2020 at 19:33

pifor · Accepted Answer · 2020-04-17 07:28:17Z

Yes Postgresql does index NULL values.

Here is a small test case:

select version();
                                                  version                                                  
-----------------------------------------------------------------------------------------------------------
 PostgreSQL 9.5.21 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39), 64-bit
(1 row)

create table t(c1 serial, c2 text);
CREATE TABLE

insert into t(c2) select generate_series(1,1000000);
INSERT 0 1000000

create index on t(c2);
CREATE INDEX

analyze t;
ANALYZE

update t set c2=null where c1=123456;
UPDATE 1

explain analyze select count(*) from t where c2 is null;
                                                      QUERY PLAN                                                

----------------------------------------------------------------------------------------------------------------
-------
 Aggregate  (cost=5.76..5.77 rows=1 width=0) (actual time=0.009..0.009 rows=1 loops=1)
   ->  Index Only Scan using t_c2_idx on t  (cost=0.42..5.76 rows=1 width=0) (actual time=0.006..0.006 rows=1 lo
ops=1)
         Index Cond: (c2 IS NULL)
         Heap Fetches: 1
 Planning time: 0.271 ms
 Execution time: 0.035 ms
(6 rows)

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