Slow index creation

[Paul van der Linden <paul.doskabouter@xxxxxxxxx>]

Hi,

I have 2 functions:

CREATE FUNCTION foo(a text, b text, c text) RETURNS text AS
$func$
DECLARE
    retVal text;
BEGIN
    SELECT
      CASE
        WHEN a='v1' AND b='b1' THEN 'r1'
        WHEN a='v1' THEN 'r2'
        ... snip long list containing various tests on a,b and c
        WHEN a='v50' THEN 'r50'
      END INTO retval;
    RETURN retVal;
END
$func$ LANGUAGE PLPGSQL IMMUTABLE;

CREATE FUNCTION bar(r text, geom geometry) RETURNS int AS
$func$
DECLARE
    retVal int;
BEGIN
    SELECT
      CASE
        WHEN r='r1' AND st_area(geom)>100 THEN 1
        WHEN r='r1' THEN 2
        ... snip long list containing various tests on r and st_area(geom)
        WHEN r='r50' THEN 25
      END INTO retval;
    RETURN retVal;
END
$func$ LANGUAGE PLPGSQL IMMUTABLE;

and a large table t (100M+ records) with columns a, b, c and geom running on PG 11, on spinning disks with 64GB memory and 28 cores.

When I create a simple geom index with CREATE INDEX ON t USING gist(geom) it finishes in about an hour, but when I create a partial index using these 2 functions

CREATE INDEX ON t USING gist(geom) WHERE bar(foo(a,b,c),geom)<12 it takes over 20 hours...

Is that because I'm using functions in the WHERE clause, or because my CASE lists are quite long, or both?

Is there any way to speed up that index creation? Is upgrading to a newer postgres a viable option (so the JITTER can speed up the functions)?

Paul