Weird CASE WHEN behaviour causing query to be suddenly very slow

Hi,

I have an issue with a rather large CASE WHEN and I cannot figure out why
it is so slow...

First, here is my test query :

SELECT CASE WHEN dwh_company_id = 1

THEN CASE

WHEN wv.source ~ '^$' THEN 'Not tracked'

WHEN wv.source ~ '^1$' THEN 'Not tracked1'

WHEN wv.source ~ '^2$' THEN 'Not tracked2'

WHEN wv.source ~ '^3$' THEN 'Not tracked3'

WHEN wv.source ~ '^4$' THEN 'Not tracked4'

WHEN wv.source ~ '^5$' THEN 'Not tracked5'

WHEN wv.source ~ '^6$' THEN 'Not tracked6'

WHEN wv.source ~ '^7$' THEN 'Not tracked7'

WHEN wv.source ~ '^8$' THEN 'Not tracked8'

WHEN wv.source ~ '^9$' THEN 'Not tracked9'

WHEN wv.source ~ '^10$' THEN 'Not tracked10'

WHEN wv.source ~ '^11$' THEN 'Not tracked11'

WHEN wv.source ~ '^12$' THEN 'Not tracked12'

WHEN wv.source ~ '^13$' THEN 'Not tracked13'

WHEN wv.source ~ '^14$' THEN 'Not tracked14'

WHEN wv.source ~ '^15$' THEN 'Not tracked15'

WHEN wv.source ~ '^16$' THEN 'Not tracked16'

WHEN wv.source ~ '^17$' THEN 'Not tracked17'

WHEN wv.source ~ '^18$' THEN 'Not tracked18'

WHEN wv.source ~ '^19$' THEN 'Not tracked19'

WHEN wv.source ~ '^20$' THEN 'Not tracked20'

WHEN wv.source ~ '^21$' THEN 'Not tracked21'

WHEN wv.source ~ '^22$' THEN 'Not tracked22'

WHEN wv.source ~ '^23$' THEN 'Not tracked23'

WHEN wv.source ~ '^24$' THEN 'Not tracked24'

WHEN wv.source ~ '^25$' THEN 'Not tracked25'

WHEN wv.source ~ '^26$' THEN 'Not tracked26'

WHEN wv.source ~ '^27$' THEN 'Not tracked27'

WHEN wv.source ~ '^28$' THEN 'Not tracked28'

--WHEN wv.source ~ '^29$' THEN 'Not tracked29'

WHEN wv.source ~ '^30$' THEN 'Not tracked30'

WHEN wv.source ~ '^31$' THEN 'Not tracked31'

WHEN wv.source ~ '^32$' THEN 'Not tracked32'

END

ELSE

'Others'

END as channel

FROM (

SELECT wv.id,

wv.ga_id,

split_part(wv.ga_source_medium, ' / ',
1) as source,

ga.dwh_source_id,

s.dwh_company_id

FROM marketing.web_visits wv

INNER JOIN dwh_metadata.google_analytics ga
ON ga.ga_id = wv.ga_id

INNER JOIN dwh_manager.sources s ON
ga.dwh_source_id =s.dwh_source_id

--WHERE s.dwh_company_id = 1

LIMIT 100000

) wv

This is a pretty simple case, my subquery (or CTE when using WITH
statement) should return 5 fields with more or less this structure :

Id : character(32)

Ga_id : bigint

Source : character(32)

Medium : character(32)

dwh_company_id : bigint

On top of which I apply a case when statement.

Now the weird thing is, using this query I notice a significant drop in
performance as the "case when" is getting bigger. If I run the query as if,
I get the following exec plain and execution time:

Subquery Scan on wv (cost=6.00..29098.17 rows=100000 width=36) (actual
time=0.828..22476.917 rows=100000 loops=1)

Buffers: shared hit=3136

-> Limit (cost=6.00..11598.17 rows=100000 width=58) (actual
time=0.209..133.429 rows=100000 loops=1)

Buffers: shared hit=3136

-> Hash Join (cost=6.00..1069811.24 rows=9228690 width=58)
(actual time=0.208..119.297 rows=100000 loops=1)

Hash Cond: (wv_1.ga_id = ga.ga_id)

Buffers: shared hit=3136

-> Seq Scan on web_visits wv_1 (cost=0.00..877005.78
rows=20587078 width=50) (actual time=0.004..18.412 rows=100000 loops=1)

Buffers: shared hit=3133

-> Hash (cost=5.50..5.50 rows=40 width=12) (actual
time=0.184..0.184 rows=111 loops=1)

Buckets: 1024 Batches: 1 Memory Usage: 5kB

Buffers: shared hit=3

-> Hash Join (cost=1.88..5.50 rows=40 width=12)
(actual time=0.056..0.148 rows=111 loops=1)

Hash Cond: (ga.dwh_source_id = s.dwh_source_id)

Buffers: shared hit=3

-> Seq Scan on google_analytics ga
(cost=0.00..2.89 rows=89 width=8) (actual time=0.007..0.028 rows=111
loops=1)

Buffers: shared hit=2

-> Hash (cost=1.39..1.39 rows=39 width=8)
(actual time=0.042..0.042 rows=56 loops=1)

Buckets: 1024 Batches: 1 Memory Usage:
3kB

Buffers: shared hit=1

-> Seq Scan on sources s (cost=0.00..1.39
rows=39 width=8) (actual time=0.005..0.020 rows=56 loops=1)

Buffers: shared hit=1

Planning time: 0.599 ms

Execution time: 22486.216 ms

Then try commenting out only one line in the case when and the query run 10x
faster :

Subquery Scan on wv (cost=6.00..28598.17 rows=100000 width=36) (actual
time=0.839..2460.002 rows=100000 loops=1)

Buffers: shared hit=3136

-> Limit (cost=6.00..11598.17 rows=100000 width=58) (actual
time=0.210..112.043 rows=100000 loops=1)

Buffers: shared hit=3136

-> Hash Join (cost=6.00..1069811.24 rows=9228690 width=58)
(actual time=0.209..99.513 rows=100000 loops=1)

Hash Cond: (wv_1.ga_id = ga.ga_id)

Buffers: shared hit=3136

-> Seq Scan on web_visits wv_1 (cost=0.00..877005.78
rows=20587078 width=50) (actual time=0.004..14.048 rows=100000 loops=1)

Buffers: shared hit=3133

-> Hash (cost=5.50..5.50 rows=40 width=12) (actual
time=0.184..0.184 rows=111 loops=1)

Buckets: 1024 Batches: 1 Memory Usage: 5kB

Buffers: shared hit=3

-> Hash Join (cost=1.88..5.50 rows=40 width=12)
(actual time=0.058..0.146 rows=111 loops=1)

Hash Cond: (ga.dwh_source_id = s.dwh_source_id)

Buffers: shared hit=3

-> Seq Scan on google_analytics ga
(cost=0.00..2.89 rows=89 width=8) (actual time=0.007..0.025 rows=111
loops=1)

Buffers: shared hit=2

-> Hash (cost=1.39..1.39 rows=39 width=8)
(actual time=0.042..0.042 rows=56 loops=1)

Buckets: 1024 Batches: 1 Memory Usage:
3kB

Buffers: shared hit=1

-> Seq Scan on sources s (cost=0.00..1.39
rows=39 width=8) (actual time=0.006..0.021 rows=56 loops=1)

Buffers: shared hit=1

Planning time: 0.583 ms

Execution time: 2467.484 ms

Why this drop in performance for only one (in this simple example) condition
? I do not really understand it. If I add more conditions to the query (let
say 1 or 2) it is also getting slower. And it's not a few ms, it is around 5
sec or so. (which is huge considering I only take in my example 1/500 of my
data with LIMIT.

Before we deviate from the problem I have (which is why the sudden drop of
performance) let me clarify a few things about this query :

- The purpose is not to rewrite it, with a join or whatever, the
case when actually comes from a function which is auto-generated by another
app we have

- My example is pretty simple and regex expressions could be
replaced by equals, the real case when query contains way more complicated
regex

- This is subset of my CASE WHEN, it is much bigger, I cut it at
the "bottleneck" point for this post.

Thanks a lot.

Best Regards,

Kevin