I am not sure if anyone else has submitted a patch for issue 2. I don't see any updates from the mailing list yet. I guess the mailing list has some delays, apologies if my efforts look duplicated to other people's efforts.
I have a potential patch for issue 2.
I was able to reproduce the issue, saw the performance degradation, and, with my patch (attached) I see an improvement in the REFRESH MATERIALIZED VIEW CONCURRENTLY.
The main crux of issue 2 is: when a materialized view has unique index on a nullable column, and when we did REFRESH MATERIALIZED VIEW CONCURRENTLY it would include that column in the FULL OUTER
JOIN condition used to detect changes.
The nullable column was showing severe performance degradation because NULL = NULL comparisons evaluate to NULL, making all rows appear different even when unchanged!
The fix I explored is to skip nullable columns when building the FULL OUTER JOIN conditions. Only include columns with NOT NULL constraints from unique indexes. The record equality operator (*=) is always included and handles nullable columns correctly.
Here is the output and performance improvement:
postgres=# \timing on
Timing is on.
postgres=# DROP MATERIALIZED VIEW IF EXISTS s CASCADE;
NOTICE: materialized view "s" does not exist, skipping
DROP MATERIALIZED VIEW
Time: 0.858 ms
postgres=#
postgres=# CREATE MATERIALIZED VIEW s AS SELECT generate_series as x, null as y FROM generate_series(1, 1000000);
SELECT 1000000
Time: 1076.254 ms (00:01.076)
postgres=#
postgres=# CREATE UNIQUE INDEX ON s(x);
CREATE INDEX
Time: 375.026 ms
postgres=# REFRESH MATERIALIZED VIEW CONCURRENTLY s;
REFRESH MATERIALIZED VIEW
Time: 3807.143 ms (00:03.807)
postgres=# CREATE UNIQUE INDEX ON s(y);
CREATE INDEX
Time: 331.382 ms
postgres=# REFRESH MATERIALIZED VIEW CONCURRENTLY s;
REFRESH MATERIALIZED VIEW
Time: 3636.049 ms (00:03.636)postgres=#
As we can see the REFRESH MATERIALIZED VIEW CONCURRENTLY now takes 3636.049 ms
With the current patch for issue 2, there is a trade-off.
The fix skips nullable columns from the join condition to avoid slowness when NULLs exist (9s vs 3s in testing). This may slightly slow down cases where nullable columns (unique index) never contain NULLs.
Users can restore full performance by adding the NOT NULL constraints to the column if they know there will never be any nulls on that column.
I would love to hear any feedback on this tradeoff and am happy to implement relevant changes.
