Create index for WHERE COALESCE() condition

Question

I'm using PostgreSQL V9.6.11

Table DDL:

CREATE TABLE test_c ( 
          insrt_prcs_id bigint NOT NULL, 
          updt_prcs_id bigint, src_sys_id integer NOT NULL,
          load_dttm timestamp(6) with time zone NOT NULL, 
          updt_dttm timestamp(6) without time zone);

I was trying to create an index for the query below:

SELECT * 
FROM test_c             
WHERE COALESCE(u_dttm,l_dttm) > '2020-04-10 15:29:44.596311-07'
AND   COALESCE(u_dttm,l_dttm) <= '2020-04-11 15:29:44.596311-07' 

Create index as:

create index idx_test_c  on test_c(COALESCE((updt_dttm, load_dttm)))

But the query planner is not scanning the index:

EXPLAIN ANALYZE
SELECT *            
FROM test_c             
WHERE COALESCE(u_dttm,l_dttm) > '2020-04-10 15:29:44.596311-07'
AND   COALESCE(u_dttm,l_dttm) <= '2020-04-11 15:29:44.596311-07' 

Seq Scan on test_c as test_c (cost=0..1857.08 rows=207 width=496) (actual=5.203..5.203 rows=0 loops=1)  
Filter: ((COALESCE((test_c.updt_dttm)::timestamp with time zone, test_c.load_dttm) > '2020-04-10 15:29:44.596311-07'::timestamp with time zone) AND (COALESCE((test_c.updt_dttm)::timestamp with time zone, test_c.load_dttm) <= '2020-04-11 15:29:44.596311-07'::timestamp with time zone))
Rows Removed by Filter: 41304

Why is the index scan not happening?

Answer 1

Strip the incorrect pair of parentheses:

CREATE INDEX idx_test_c ON test_c(COALESCE(updt_dttm, load_dttm));

db<>fiddle here

The way you had it was effectively indexing the composite value (updt_dttm, load_dttm), COALESCE not doing anything.

Your second problem was revealed by the added table definition:

CREATE TABLE test_c ( 
  insrt_prcs_id bigint NOT NULL
, updt_prcs_id  bigint
, src_sys_id    integer NOT NULL
, load_dttm     timestamp(6) with time zone NOT NULL
, updt_dttm     timestamp(6) without time zone   -- !!!
);

Why would you use different data types for load_dttm and updt_dttm? Fix that and the second problem goes away. I suggest:

CREATE TABLE test_c ( 
  -- ...
, load_dttm     timestamp with time zone NOT NULL
, updt_dttm     timestamp with time zone  -- !!!
);

Why?

You got this error:

ERROR: functions in index expression must be marked IMMUTABLE

.. because COALESCE must return one data type. timestamp with time zone (timestamtz) is the "preferred type", so updt_dttm timestamp is coerced to timestamptz, which uses a function that depends on the timezone setting of the current session and hence is not IMMUTABLE. And index expressions cannot involve non-IMMUTABLE functions.

Related:

• Is timestamptz preferred when timezone logic is performed by the client?
• Ignoring time zones altogether in Rails and PostgreSQL

You could make the index work with your original (broken) table design by hard-coding the time zone - 'Europe/Vienna' in my example:

CREATE INDEX ON test_c(COALESCE(updt_dttm AT TIME ZONE 'Europe/Vienna', load_dttm));

Just a proof of concept. Queries hoping to use that index would have to use the same expression. Don't go there if you don't have to. Fix your table definition instead.