*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