CREATE INDEX ON words_moves(length(letters), score) WHERE action = 'play'

Good evening,

in PostgreSQL 11 I have a table holding player moves (could be: 'play',
'swap', 'skip', ...) in a word game:

# \d words_moves;
Table "public.words_moves"
Column | Type | Collation | Nullable |
Default
---------+--------------------------+-----------+----------+------------------------------------------
mid | bigint | | not null |
nextval('words_moves_mid_seq'::regclass)
action | text | | not null |
gid | integer | | not null |
uid | integer | | not null |
played | timestamp with time zone | | not null |
tiles | jsonb | | |
score | integer | | |
letters | text | | |
hand | text | | |
puzzle | boolean | | not null | false
Indexes:
"words_moves_pkey" PRIMARY KEY, btree (mid)
"words_moves_gid_played_idx" btree (gid, played DESC)
"words_moves_uid_action_played_idx" btree (uid, action, played)
"words_moves_uid_idx" btree (uid)
Check constraints:
"words_moves_score_check" CHECK (score >= 0)
Foreign-key constraints:
"words_moves_gid_fkey" FOREIGN KEY (gid) REFERENCES words_games(gid) ON
DELETE CASCADE
"words_moves_uid_fkey" FOREIGN KEY (uid) REFERENCES words_users(uid) ON
DELETE CASCADE
Referenced by:
TABLE "words_scores" CONSTRAINT "words_scores_mid_fkey" FOREIGN KEY
(mid) REFERENCES words_moves(mid) ON DELETE CASCADE

When I search for "interesting moves" with score higher than 90 or all 7
tiles played, then the query takes a bit longer:

EXPLAIN ANALYZE
SELECT
TO_CHAR(played, 'Mon YYYY'),
COUNT(NULLIF(puzzle, FALSE)) OVER (PARTITION BY TO_CHAR(played, 'Mon
YYYY')),
puzzle,
mid,
MD5(mid || 'cookie'),
gid,
score
FROM words_moves
WHERE action = 'play'
AND LENGTH(hand) = 7
AND (LENGTH(letters) = 7 OR score > 90)
AND played > CURRENT_TIMESTAMP - interval '1 year'
AND played < CURRENT_TIMESTAMP - interval '3 day'
ORDER BY played DESC;

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=168533.19..168533.30 rows=44 width=97) (actual
time=2126.433..2126.541 rows=1036 loops=1)
Sort Key: played DESC
Sort Method: quicksort Memory: 194kB
-> WindowAgg (cost=168530.56..168531.99 rows=44 width=97) (actual
time=2122.991..2125.593 rows=1036 loops=1)
-> Sort (cost=168530.56..168530.67 rows=44 width=57) (actual
time=2122.934..2123.049 rows=1036 loops=1)
Sort Key: (to_char(played, 'Mon YYYY'::text))
Sort Method: quicksort Memory: 129kB
-> Gather (cost=1000.00..168529.36 rows=44 width=57)
(actual time=287.461..2121.445 rows=1036 loops=1)
Workers Planned: 2
Workers Launched: 2
-> Parallel Seq Scan on words_moves
(cost=0.00..167524.96 rows=18 width=57) (actual time=207.234..2115.686
rows=345 loops=3)
Filter: ((action = 'play'::text) AND
(length(hand) = 7) AND ((length(letters) = 7) OR (score > 90)) AND (played
> (CURRENT_TIMESTAMP - '1 year'::interval)) AND (played <
(CURRENT_TIMESTAMP - '3 days'::interval)))
Rows Removed by Filter: 1088073
Planning Time: 0.383 ms
Execution Time: 2126.922 ms
(15 rows)

Here the link: https://explain.depesz.com/s/s3HF

So I add an index with -

CREATE INDEX ON words_moves(length(letters), score, action);

That is because I have several such queries looking for interesting 'play'
moves with high score or all 7 tiles played.

This helps a bit:

QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=126487.34..126487.45 rows=44 width=97) (actual
time=219.649..219.744 rows=1036 loops=1)
Sort Key: played DESC
Sort Method: quicksort Memory: 194kB
-> WindowAgg (cost=126484.71..126486.14 rows=44 width=97) (actual
time=216.180..218.680 rows=1036 loops=1)
-> Sort (cost=126484.71..126484.82 rows=44 width=57) (actual
time=216.125..216.226 rows=1036 loops=1)
Sort Key: (to_char(played, 'Mon YYYY'::text))
Sort Method: quicksort Memory: 129kB
-> Bitmap Heap Scan on words_moves
(cost=83892.69..126483.50 rows=44 width=57) (actual time=209.447..214.879
rows=1036 loops=1)
Recheck Cond: (((length(letters) = 7) AND (action =
'play'::text)) OR ((score > 90) AND (action = 'play'::text)))
Filter: ((length(hand) = 7) AND (played >
(CURRENT_TIMESTAMP - '1 year'::interval)) AND (played < (CURRENT_TIMESTAMP
- '3 days'::interval)))
Rows Removed by Filter: 468
Heap Blocks: exact=1495
-> BitmapOr (cost=83892.69..83892.69 rows=15280
width=0) (actual time=209.083..209.083 rows=0 loops=1)
-> Bitmap Index Scan on
words_moves_length_score_action_idx (cost=0.00..419.69 rows=14838 width=0)
(actual time=2.040..2.040 rows=912 loops=1)
Index Cond: ((length(letters) = 7) AND
(action = 'play'::text))
-> Bitmap Index Scan on
words_moves_length_score_action_idx (cost=0.00..83472.98 rows=442 width=0)
(actual time=207.038..207.038 rows=608 loops=1)
Index Cond: ((score > 90) AND (action =
'play'::text))
Planning Time: 1.102 ms
Execution Time: 220.676 ms
(19 rows)

Here the resulting link: https://explain.depesz.com/s/Pwbt

Then I drop that index and create another one:

CREATE INDEX ON words_moves(length(letters), score) WHERE action = 'play';

And it seems to have a better performance on the query:

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=97687.61..97687.72 rows=44 width=97) (actual
time=175.832..175.927 rows=1036 loops=1)
Sort Key: played DESC
Sort Method: quicksort Memory: 194kB
-> WindowAgg (cost=97684.98..97686.41 rows=44 width=97) (actual
time=172.443..174.937 rows=1036 loops=1)
-> Sort (cost=97684.98..97685.09 rows=44 width=57) (actual
time=172.390..172.490 rows=1036 loops=1)
Sort Key: (to_char(played, 'Mon YYYY'::text))
Sort Method: quicksort Memory: 129kB
-> Bitmap Heap Scan on words_moves
(cost=55092.96..97683.78 rows=44 width=57) (actual time=165.420..171.164
rows=1036 loops=1)
Recheck Cond: (((length(letters) = 7) AND (action =
'play'::text)) OR ((score > 90) AND (action = 'play'::text)))
Filter: ((length(hand) = 7) AND (played >
(CURRENT_TIMESTAMP - '1 year'::interval)) AND (played < (CURRENT_TIMESTAMP
- '3 days'::interval)))
Rows Removed by Filter: 468
Heap Blocks: exact=1495
-> BitmapOr (cost=55092.96..55092.96 rows=15280
width=0) (actual time=165.036..165.036 rows=0 loops=1)
-> Bitmap Index Scan on
words_moves_length_score_idx (cost=0.00..275.71 rows=14838 width=0)
(actual time=0.620..0.620 rows=912 loops=1)
Index Cond: (length(letters) = 7)
-> Bitmap Index Scan on
words_moves_length_score_idx (cost=0.00..54817.23 rows=442 width=0)
(actual time=164.413..164.413 rows=608 loops=1)
Index Cond: (score > 90)
Planning Time: 0.948 ms
Execution Time: 177.604 ms
(19 rows)

Here the resulting link: https://explain.depesz.com/s/pmCw

I still wonder, what am I missing, what could be improved there?

Does anybody please have a hint?

Thanks
Alex