Message-ID: <1eac0b83-217d-94d2-cde2-db74ae3fbe86@posteo.de>
Date: Fri, 12 Aug 2022 18:49:58 +0000
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To: pgsql-performance@lists.postgresql.org
From: Nico Heller <nico.heller@posteo.de>
Subject: to_jsonb performance on array aggregated correlated subqueries
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Good day,

consider the following query:

WITH aggregation(
     SELECT
            a.*,
           (SELECT array_agg(b.*) FROM b WHERE b.a_id = a.id) as "bs",
           (SELECT array_agg(c.*) FROM c WHERE c.a_id = a.id) as "cs",
           (SELECT array_agg(d.*) FROM d WHERE d.a_id = a.id) as "ds",
           (SELECT array_agg(e.*) FROM d WHERE e.a_id = a.id) as "es"
     FROM a WHERE a.id IN (<some big list, ranging from 20-180 entries)
)
SELECT to_jsonb(aggregation.*) as "value" FROM aggregation;

Imagine that for each "a" there exists between 5-100 "b", "c", "d" and 
"e" which makes the result of this pretty big (worst case: around 300kb 
when saved to a text file).
I noticed that adding the "to_jsonb" increases the query time by 100%, 
from 9-10ms to 17-23ms on average.
This may not seem slow at all but this query has another issue: on an 
AWS Aurora Serverless V2 instance we are running into a RAM usage of 
around 30-50 GB compared to < 10 GB when using a simple LEFT JOINed 
query when under high load (> 1000 queries / sec). Furthermore the CPU 
usage is quite high.

Is there anything I could improve? I am open for other solutions but I 
am wondering if I ran into an edge case of "to_jsonb" for "anonymous 
records" (these are just rows without a defined UDT) - this is just a 
wild guess though.
I am mostly looking to decrease the load (CPU and memory) on Postgres 
itself. Furthermore I would like to know why the memory usage is so 
significant. Any tips on how to analyze this issue are appreciated as 
well -  my knowledge is limited to being average at interpreting EXPLAIN 
ANALYZE results.

Here's a succinct list of the why's, what I have found out so far and 
solution I already tried/ don't want to consider:

- LEFT JOINing potentially creates a huge resultset because of the 
cartesian product, thats a nono
- not using "to_jsonb" is sadly also not possible as Postgres' array + 
record syntax is very unfriendly and hard to parse (it's barely 
documented if at all and the quoting rules are cumbersome, furthermore I 
lack column names in the array which would make the parsing sensitive to 
future table changes and thus cumbersome to maintain) in my application
- I know I could solve this with a separate query for a,b,c,d and e 
while "joinining" the result in my application, but I am looking for 
another way to do this (bear with me, treat this as an academic question :))
- I am using "to_jsonb" to simply map the result to my data model via a 
json mapper
- EXPLAIN ANALYZE is not showing anything special when using "to_jsonb" 
vs. not using it, the outermost (hash) join just takes more time - is 
there a more granular EXPLAIN that shows me the runtime of functions 
like "to_jsonb"?
- I tried an approach where b,c,d,e where array columns of UDTs: UDTs 
are not well supported by my application stack (JDBC) and are generally 
undesireable for me (because of a lack of migration possibilities)
- I don't want to duplicate my data into another table (e.g. that has 
jsonb columns)
- MATERIALIZED VIEWS are also undesirable as the manual update, its 
update is non-incremental which would make a refresh on a big data set 
take a long time
- split the query into chunks to reduce the IN()-statement list size 
makes no measurable difference
- I don't want to use JSONB columns for b,c,d and e because future 
changes of b,c,d or e's structure (e.g. new fields, changing a datatype) 
are harder to achieve with JSONB and it lacks constraint checks on 
insert (e.g. not null on column b.xy)

Kind regards and thank you for your time,
Nico Heller

P.S: Sorry for the long list of "I don't want to do this", some of them 
are not possible because of other requirements