Unexplainable execution time difference between two test functions...one using IF (SELECT COUNT(*) FROM...) and the other using IF EXISTS (SELECT 1 FROM...)

*A description of what you are trying to achieve and what results you
expect.:*

My end goal was to test the execution time difference between using an
IF(SELECT COUNT(*)...) and an IF EXISTS() when no indexes were used and
when a string match was not found. My expectation was that my 2 functions
would behave fairly similarly, but they most certainly did not. Here are
the table, functions, test queries, and test query results I received, as
well as comments as I present the pieces and talk about the results from my
perspective.

This is the table and data that I used for my tests. A table with 1
million sequenced records. No indexing on any columns. I ran ANALYZE on
this table and a VACUUM on the entire database, just to be sure.

CREATE TABLE zz_noidx1 AS SELECT generate_series(0, 999999) AS
int_distinct, 'Test'::text || generate_series(0, 999999)::text AS
text_distinct;

These are the 2 functions that I ran my final tests with. My goal was to
determine which function would perform the fastest and my expectation was
that they would still be somewhat close in execution time comparison.

--Test Function #1
CREATE OR REPLACE FUNCTION zz_spx_ifcount_noidx(p_findme text)
RETURNS text
LANGUAGE 'plpgsql'
STABLE
AS $$

BEGIN
IF (SELECT COUNT(*) FROM zz_noidx1 WHERE LOWER(zz_noidx1.text_distinct) =
LOWER(p_findme)) > 0 THEN
RETURN 'Found';
ELSE
RETURN 'Not Found';
END IF;
END;
$$;

--Test Function #2
CREATE OR REPLACE FUNCTION zz_spx_ifexists_noidx(p_findme text)
RETURNS text
LANGUAGE 'plpgsql'
STABLE
AS $$

BEGIN
IF EXISTS (SELECT 1 FROM zz_noidx1 WHERE LOWER(zz_noidx1.text_distinct) =
LOWER(p_findme)) THEN
RETURN 'Found';
ELSE
RETURN 'Not Found';
END IF;
END;
$$;

The first thing I did was to run some baseline tests using the basic
queries inside of the IF() checks found in each of the functions to see how
the query planner handled them. I ran the following two queries.

EXPLAIN (ANALYZE, BUFFERS) SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test5000001');
EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test5000001');

The execution time results and query plans for these two were very similar,
as expected. In the results I can see that 2 workers were employed for
each query plan.

--Results for the SELECT COUNT(*) query.
QUERY
PLAN

----------------------------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=12661.42..12661.43 rows=1 width=8) (actual
time=172.105..172.105 rows=1 loops=1)
Buffers: shared
read=1912

-> Gather (cost=12661.21..12661.42 rows=2 width=8) (actual
time=172.020..172.099 rows=3 loops=1)
Workers Planned:
2

Workers Launched:
2

Buffers: shared
read=1912

-> Partial Aggregate (cost=11661.21..11661.22 rows=1 width=8) (actual
time=155.123..155.123 rows=1 loops=3)
Buffers: shared
read=5406

-> Parallel Seq Scan on zz_noidx1 (cost=0.00..11656.00 rows=2083
width=0) (actual time=155.103..155.103 rows=0 loops=3)
Filter: (lower(text_distinct) =
'test5000001'::text)

Rows Removed by Filter:
333333

Buffers: shared
read=5406

Planning time: 0.718
ms

Execution time: 187.601 ms

--Results for the SELECT 1 query.
QUERY
PLAN

----------------------------------------------------------------------------------------------------------------------------
Gather (cost=1000.00..13156.00 rows=5000 width=4) (actual
time=175.682..175.682 rows=0 loops=1)
Workers Planned:
2

Workers Launched:
2

Buffers: shared
read=2021

-> Parallel Seq Scan on zz_noidx1 (cost=0.00..11656.00 rows=2083
width=4) (actual time=159.769..159.769 rows=0 loops=3)
Filter: (lower(text_distinct) =
'test5000001'::text)

Rows Removed by Filter:
333333

Buffers: shared
read=5406

Planning time: 0.874
ms

Execution time: 192.045 ms

After running these baseline tests and viewing the fairly similar results,
right or wrong, I expected my queries that tested the functions to behave
similarly. I started with the following query...

EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM
zz_spx_ifcount_noidx('Test5000001');

and I got the following "auto_explain" results...

2018-04-16 14:57:22.624 EDT [17812] LOG: duration: 155.239 ms plan:
Query Text: SELECT (SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme)) > 0
Partial Aggregate (cost=11661.21..11661.22 rows=1 width=8) (actual
time=155.230..155.230 rows=1 loops=1)
Buffers: shared read=1682
-> Parallel Seq Scan on zz_noidx1 (cost=0.00..11656.00 rows=2083
width=0) (actual time=155.222..155.222 rows=0 loops=1)
Filter: (lower(text_distinct) = 'test5000001'::text)
Rows Removed by Filter: 311170
Buffers: shared read=1682
2018-04-16 14:57:22.624 EDT [9096] LOG: duration: 154.603 ms plan:
Query Text: SELECT (SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme)) > 0
Partial Aggregate (cost=11661.21..11661.22 rows=1 width=8) (actual
time=154.576..154.576 rows=1 loops=1)
Buffers: shared read=1682
-> Parallel Seq Scan on zz_noidx1 (cost=0.00..11656.00 rows=2083
width=0) (actual time=154.570..154.570 rows=0 loops=1)
Filter: (lower(text_distinct) = 'test5000001'::text)
Rows Removed by Filter: 311061
Buffers: shared read=1682
2018-04-16 14:57:22.642 EDT [15132] LOG: duration: 197.260 ms plan:
Query Text: SELECT (SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme)) > 0
Result (cost=12661.43..12661.45 rows=1 width=1) (actual
time=179.561..179.561 rows=1 loops=1)
Buffers: shared read=2042
InitPlan 1 (returns $1)
-> Finalize Aggregate (cost=12661.42..12661.43 rows=1 width=8) (actual
time=179.559..179.559 rows=1 loops=1)
Buffers: shared read=2042
-> Gather (cost=12661.21..12661.42 rows=2 width=8) (actual
time=179.529..179.556 rows=3 loops=1)
Workers Planned: 2
Workers Launched: 2
Buffers: shared read=2042
-> Partial Aggregate (cost=11661.21..11661.22 rows=1 width=8)
(actual time=162.831..162.831 rows=1 loops=3)
Buffers: shared read=5406
-> Parallel Seq Scan on zz_noidx1 (cost=0.00..11656.00 rows=2083
width=0) (actual time=162.824..162.824 rows=0 loops=3)
Filter: (lower(text_distinct) = 'test5000001'::text)
Rows Removed by Filter: 333333
Buffers: shared read=5406
2018-04-16 14:57:22.642 EDT [15132] CONTEXT: SQL statement "SELECT (SELECT
COUNT(*) FROM zz_noidx1 WHERE LOWER(zz_noidx1.text_distinct) =
LOWER(p_findme)) > 0"
PL/pgSQL function zz_spx_ifcount_noidx(text) line 4 at IF
2018-04-16 14:57:22.642 EDT [15132] LOG: duration: 199.371 ms plan:
Query Text: explain (analyze, buffers) select * from
zz_spx_ifcount_noidx('Test5000001')
Function Scan on zz_spx_ifcount_noidx (cost=0.25..0.26 rows=1 width=32)
(actual time=199.370..199.370 rows=1 loops=1)
Buffers: shared hit=218 read=5446

Here I could see that the 2 workers were getting employed again, which is
great. Just what I expected. And the execution time was in the same
ballpark as my first baseline test using just the query found inside of the
IF() check. 199 milliseonds. Okay.

I moved on to test the other function with the following query...

EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM
zz_spx_ifcount_noidx('Test5000001');

and I got the following "auto_explain" results...

2018-04-16 14:58:34.134 EDT [12616] LOG: duration: 426.279 ms plan:
Query Text: SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))
Result (cost=4.08..4.09 rows=1 width=1) (actual time=426.274..426.274
rows=1 loops=1)
Buffers: shared read=5406
InitPlan 1 (returns $0)
-> Seq Scan on zz_noidx1 (cost=0.00..20406.00 rows=5000 width=0)
(actual time=426.273..426.273 rows=0 loops=1)
Filter: (lower(text_distinct) = 'test5000001'::text)
Rows Removed by Filter: 1000000
Buffers: shared read=5406
2018-04-16 14:58:34.134 EDT [12616] CONTEXT: SQL statement "SELECT EXISTS
(SELECT 1 FROM zz_noidx1 WHERE LOWER(zz_noidx1.text_distinct) =
LOWER(p_findme))"
PL/pgSQL function zz_spx_ifexists_noidx(text) line 4 at IF
2018-04-16 14:58:34.134 EDT [12616] LOG: duration: 428.077 ms plan:
Query Text: explain (analyze, buffers) select * from
zz_spx_ifexists_noidx('Test5000001')
Function Scan on zz_spx_ifexists_noidx (cost=0.25..0.26 rows=1 width=32)
(actual time=428.076..428.076 rows=1 loops=1)
Buffers: shared hit=30 read=5438

Definitely not the execution time, or query plan, results I was expecting.
As we can see, no workers were employed here and my guess was that this was
the reason or the large execution time difference between these 2 tests?
199 milliseconds versus 428 milliseconds, which is a big difference. Why
are workers not being employed here like they were when I tested the query
found inside of the IF() check in a standalone manner? But then I ran
another test and the results made even less sense to me.

When I ran the above query the first 5 times after starting my Postgres
service, I got the same results each time (around 428 milliseconds), but
when running the query 6 or more times, the execution time jumps up to
almost double that. Here are the "auto_explain" results running this query
a 6th time...

--"auto_explain" results after running the same query 6 or more times.
2018-04-16 15:01:51.635 EDT [12616] LOG: duration: 761.847 ms plan:
Query Text: SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))
Result (cost=4.58..4.59 rows=1 width=1) (actual time=761.843..761.843
rows=1 loops=1)
Buffers: shared hit=160 read=5246
InitPlan 1 (returns $0)
-> Seq Scan on zz_noidx1 (cost=0.00..22906.00 rows=5000 width=0)
(actual time=761.841..761.841 rows=0 loops=1)
Filter: (lower(text_distinct) = lower($1))
Rows Removed by Filter: 1000000
Buffers: shared hit=160 read=5246
2018-04-16 15:01:51.635 EDT [12616] CONTEXT: SQL statement "SELECT EXISTS
(SELECT 1 FROM zz_noidx1 WHERE LOWER(zz_noidx1.text_distinct) =
LOWER(p_findme))"
PL/pgSQL function zz_spx_ifexists_noidx(text) line 4 at IF
2018-04-16 15:01:51.635 EDT [12616] LOG: duration: 762.156 ms plan:
Query Text: explain (analyze, buffers) select * from
zz_spx_ifexists_noidx('Test5000001')
Function Scan on zz_spx_ifexists_noidx (cost=0.25..0.26 rows=1 width=32)
(actual time=762.154..762.155 rows=1 loops=1)
Buffers: shared hit=160 read=5246

As you can see, the execution time jumps up to about 762 milliseonds. I
can see in the sequence scan node that the LOWER() function shows up on the
right side of the equal operator, whereas in the first 5 runs of this test
query the plan did not show this. Why is this?

I tried increasing the "work_mem" setting to 1GB to see if this made any
difference, but the results were the same.

So those were the tests that I performed and the results I received, which
left me with many questions. If anyone is able to help me understand this
behavior, I'd greatly appreciate it. This is my first post to the email
list, so I hope I did a good enough job providing all the information
needed.

Thanks!
Ryan

*PostgreSQL version number you are running:*

PostgreSQL 10.2, compiled by Visual C++ build 1800, 64-bit

*How you installed PostgreSQL:*

Using the Enterprise DB installer.

I have also installed Enterprise DB's Postgres Enterprise Manager (PEM)
7.2.0 software and Enterprise DB's SQL Profiler PG10-7.2.0 software. The
PEM Agent service that gets installed is currently turned off.

*Changes made to the settings in the postgresql.conf file: see Server
Configuration for a quick way to list them all.*

name |current_setting
|source
-----------------------------------|---------------------------------------|---------------------
application_name |DBeaver 5.0.3 - Main
|session
auto_explain.log_analyze |on
|configuration file
auto_explain.log_buffers |on
|configuration file
auto_explain.log_min_duration |0
|configuration file
auto_explain.log_nested_statements |on
|configuration file
auto_explain.log_triggers |on
|configuration file
client_encoding |UTF8
|client
DateStyle |ISO, MDY
|client
default_text_search_config |pg_catalog.english
|configuration file
dynamic_shared_memory_type |windows
|configuration file
extra_float_digits |3
|session
lc_messages |English_United States.1252
|configuration file
lc_monetary |English_United States.1252
|configuration file
lc_numeric |English_United States.1252
|configuration file
lc_time |English_United States.1252
|configuration file
listen_addresses |*
|configuration file
log_destination |stderr
|configuration file
log_timezone |US/Eastern
|configuration file
logging_collector |on
|configuration file
max_connections |100
|configuration file
max_stack_depth |2MB
|environment variable
port |5432
|configuration file
shared_buffers |128MB
|configuration file
shared_preload_libraries |$libdir/sql-profiler.dll, auto_explain
|configuration file
ssl |on
|configuration file
ssl_ca_file |root.crt
|configuration file
ssl_cert_file |server.crt
|configuration file
ssl_crl_file |root.crl
|configuration file
ssl_key_file |server.key
|configuration file
TimeZone |America/New_York
|client

*Operating system and version:*

Windows 10 Pro 64-bit, Version 1709 (Build 16299.309)

*Hardware:*

Processor - Intel Core i7-7820HQ @ 2.90GHz
RAM - 16GB
RAID? - No
Hard Drive - Samsung 512 GB SSD M.2 PCIe NVMe Opal2

*What program you're using to connect to PostgreSQL:*

DBeaver Community Edition v5.0.3

*Is there anything relevant or unusual in the PostgreSQL server logs?:*

Not that I noticed.

*For questions about any kind of error:*

N/A

*What you were doing when the error happened / how to cause the error:*

N/A

*The EXACT TEXT of the error message you're getting, if there is one: (Copy
and paste the message to the email, do not send a screenshot)*

N/A