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

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

2018-04-17 Thread Tomas Vondra

On 04/17/2018 05:43 PM, Hackety Man wrote:
> 
> 
> On Tue, Apr 17, 2018 at 10:23 AM, Tomas Vondra
> > wrote:
> 
> 
> 
> On 04/17/2018 04:01 PM, Hackety Man wrote:
> 
>  ...
> Right.  I was more wondering why it switched over to a generic
> plan, as you've stated, like clockwork starting with the 6th
> execution run.
> 
> 
> That's a hard-coded value. The first 5 executions are re-planned
> using the actual parameter values, and then we try generating a
> generic plan and see if it's cheaper than the non-generic one. You
> can disable that, though.
> 
> 
> 
> So on that note, in the planner's eyes, starting with the 6th execution,
> it looks like the planner still thinks that the generic plan will
> perform better than the non-generic one, which is why it keeps using the
> generic plan from that point forward?
> 

Yes. The point of prepared statements (which also applies to plpgsql, as
it uses prepared statements automatically) is to eliminate the planning
overhead. So we try planning it with actual parameter values for the
first 5 plans, and then compare it to the generic plan.

> Similar to the parallel scans, any chance of the planner possibly being
> enhanced in the future to come to a better conclusion as to whether, or
> not, the generic plan will perform better than the non-generic plan?  :-)

There's always hope, but it's hard to say if/when an enhancement will
happen, unfortunately.

regards

-- 
Tomas Vondra  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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

2018-04-17 Thread Hackety Man

On Tue, Apr 17, 2018 at 10:23 AM, Tomas Vondra  wrote:

>
>
> On 04/17/2018 04:01 PM, Hackety Man wrote:
>
>>
>>
>> On Tue, Apr 17, 2018 at 6:49 AM, Tomas Vondra <
>> tomas.von...@2ndquadrant.com >
>> wrote:
>>
>>
>>
>> On 04/16/2018 10:42 PM, Hackety Man wrote:
>>
>> ...
>>  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('Test501');
>>  EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
>>  LOWER(text_distinct) = LOWER('Test501');
>>
>>
>> Those are not the interesting plans, though. The EXISTS only cares
>> about the first row, so you should be looking at
>>
>>  EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
>>  LOWER(text_distinct) = LOWER('Test501') LIMIT 1;
>>
>>
>>
>> Okay.  I tested this query and it did return an execution time on par
>> with my tests of the "zz_spx_ifexists_noidx" function.
>> *
>> *
>>
>>
>>
>>
>>  I moved on to test the other function with the following
>> query...
>>
>>  EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM
>>  zz_spx_ifcount_noidx('Test501');
>>
>>  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) =
>> 'test501'::text)
>>Rows Removed by Filter: 100
>>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('Test501')
>>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.
>>
>>
>> The plan difference is due to not realizing the EXISTS essentially
>> implies LIMIT 1. Secondly, it expects about 5000 rows matching the
>> condition,  uniformly spread through the table. But it apparently
>> takes much longer to find the first one, hence the increased duration.
>>
>>
>>
>> Ah.  I did not know that.  So EXISTS inherently applies a LIMIT 1, even
>> though it doesn't show in the query plan, correct? Is it not possible for
>> parallel scans to be implemented while applying an implicit, or explicit,
>> LIMIT 1?
>> **//___^
>>
>>
> It doesn't add a limit node to the plan, but it behaves similarly to that.
> The database only needs to fetch the first row to answer the EXISTS
> predicate.
>
> I don't think this is inherently incompatible with parallel plans, but the
> planner simply thinks it's going to bee very cheap - cheaper than setting
> up parallel workers etc. So it does not do that.



Understood.  Any chance of the planner possibly being

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

2018-04-17 Thread Pavel Stehule

2018-04-17 12:52 GMT+02:00 Tomas Vondra :

>
>
> On 04/17/2018 07:17 AM, Pavel Stehule wrote:
>
>> Hi
>>
>> 2018-04-16 22:42 GMT+02:00 Hackety Man  hackety...@gmail.com>>:
>>
>> ...
>>
> >
>
>> A support of parallel query execution is not complete -  it doesn't work
>> in PostgreSQL 11 too. So although EXISTS variant can be faster (but can be
>> - the worst case of EXISTS is same like COUNT), then due disabled parallel
>> execution the COUNT(*) is faster now. It is unfortunate, because I believe
>> so this issue will be fixed in few years.
>>
>>
> None of the issues seems to be particularly related to parallel query.
> It's much more likely a general issue with planning EXISTS / LIMIT and
> non-uniform data distribution.


I was wrong EXISTS are not supported. It looks like new dimension of
performance issues related to parallelism. I understand so this example is
worst case.

postgres=# EXPLAIN (ANALYZE, BUFFERS) select exists(SELECT * FROM zz_noidx1
WHERE LOWER(text_distinct) = LOWER('Test501'));
  QUERY
PLAN
--
 Result  (cost=4.08..4.09 rows=1 width=1) (actual time=423.600..423.600
rows=1 loops=1)
   Buffers: shared hit=3296 read=2110
   InitPlan 1 (returns $0)
 ->  Seq Scan on zz_noidx1  (cost=0.00..20406.00 rows=5000 width=0)
(actual time=423.595..423.595 rows=0 loops=1)
   Filter: (lower(text_distinct) = 'test501'::text)
   Rows Removed by Filter: 100
   Buffers: shared hit=3296 read=2110
 Planning Time: 0.133 ms
 Execution Time: 423.633 ms

postgres=# EXPLAIN (ANALYZE, BUFFERS) SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test501');
   QUERY
PLAN
-
 Finalize Aggregate  (cost=12661.42..12661.43 rows=1 width=8) (actual
time=246.662..246.662 rows=1 loops=1)
   Buffers: shared hit=817 read=549
   ->  Gather  (cost=12661.21..12661.42 rows=2 width=8) (actual
time=246.642..246.656 rows=3 loops=1)
 Workers Planned: 2
 Workers Launched: 2
 Buffers: shared hit=817 read=549
 ->  Partial Aggregate  (cost=11661.21..11661.22 rows=1 width=8)
(actual time=242.168..242.169 rows=1 loops=3)
   Buffers: shared hit=3360 read=2046
   ->  Parallel Seq Scan on zz_noidx1  (cost=0.00..11656.00
rows=2083 width=0) (actual time=242.165..242.165 rows=0 loops=3)
 Filter: (lower(text_distinct) = 'test501'::text)
 Rows Removed by Filter: 33
 Buffers: shared hit=3360 read=2046
 Planning Time: 0.222 ms
 Execution Time: 247.927 ms

The cost of EXISTS is too low to use parallelism, and value is found too
late.

When I decrease startup cost to 0 of parallel exec I got similar plan,
similar time

postgres=# EXPLAIN (ANALYZE, BUFFERS) select exists(SELECT * FROM zz_noidx1
WHERE LOWER(text_distinct) = LOWER('Test501'));
 QUERY
PLAN
-
 Result  (cost=2.43..2.44 rows=1 width=1) (actual time=246.398..246.402
rows=1 loops=1)
   Buffers: shared hit=885 read=489
   InitPlan 1 (returns $1)
 ->  Gather  (cost=0.00..12156.00 rows=5000 width=0) (actual
time=246.393..246.393 rows=0 loops=1)
   Workers Planned: 2
   Workers Launched: 2
   Buffers: shared hit=885 read=489
   ->  Parallel Seq Scan on zz_noidx1  (cost=0.00..11656.00
rows=2083 width=0) (actual time=241.067..241.067 rows=0 loops=3)
 Filter: (lower(text_distinct) = 'test501'::text)
 Rows Removed by Filter: 33
 Buffers: shared hit=3552 read=1854
 Planning Time: 0.138 ms
 Execution Time: 247.623 ms
(13 rows)

>From this perspective it looks so cost of EXISTS(subselect) is maybe too
low.

Regards

Pavel






>
>
> regards
>
> --
> Tomas Vondra  http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>

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

2018-04-17 Thread Hackety Man

Hi Pavel,

Thanks for sharing that information.  I was not aware that the parallel
query functionality was not yet fully implemented.

Thanks,
Ryan

On Tue, Apr 17, 2018 at 1:17 AM, Pavel Stehule 
wrote:

> Hi
>
> 2018-04-16 22:42 GMT+02:00 Hackety Man :
>
>> *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, 99) AS
>> int_distinct, 'Test'::text || generate_series(0, 99)::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('Test501');
>> EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
>> LOWER(text_distinct) = LOWER('Test501');
>>
>> 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) = 'test501'::text)
>>
>>  Rows Removed by Filter: 33
>>
>>  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) = 'test501'::text)
>>
>>   Rows Removed by Filter: 33
>>
>>   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('Test5000
>> 001');
>>
>> and I got the following "auto_explain" results...
>>
>> 2018-04-16 14:57:22.624 EDT [17812] LOG:  duration:

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

2018-04-17 Thread Hackety Man

On Tue, Apr 17, 2018 at 6:49 AM, Tomas Vondra 
wrote:

>
>
> On 04/16/2018 10:42 PM, Hackety Man wrote:
>
>> ...
>> 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('Test501');
>> EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
>> LOWER(text_distinct) = LOWER('Test501');
>>
>>
> Those are not the interesting plans, though. The EXISTS only cares about
> the first row, so you should be looking at
>
> EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
> LOWER(text_distinct) = LOWER('Test501') LIMIT 1;



Okay.  I tested this query and it did return an execution time on par with
my tests of the "zz_spx_ifexists_noidx" function.



>
>
> I moved on to test the other function with the following query...
>>
>> EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM
>> zz_spx_ifcount_noidx('Test501');
>>
>> 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) = 'test501'::text)
>>   Rows Removed by Filter: 100
>>   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('Test501')
>>   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.
>>
>>
> The plan difference is due to not realizing the EXISTS essentially implies
> LIMIT 1. Secondly, it expects about 5000 rows matching the condition,
> uniformly spread through the table. But it apparently takes much longer to
> find the first one, hence the increased duration.
>


Ah.  I did not know that.  So EXISTS inherently applies a LIMIT 1, even
though it doesn't show in the query plan, correct?  Is it not possible for
parallel scans to be implemented while applying an implicit, or explicit,
LIMIT 1?



>
> How did you generate the data?



I used generate_series() to create 1 million records in sequence at the
same time that I created the table using the following script...

CREATE TABLE zz_noidx1 AS SELECT generate_series(0, 99) AS
int_distinct, 'Test'::text || generate_series(0, 99)::text AS
text_distinct;


>
> 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...
>>
>>
> This is likely due to generating a generic plan after the fifth execution.
> There seems to be only small difference in costs, though.
>
>
> --"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
>>

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

2018-04-16 Thread Pavel Stehule

Hi

2018-04-16 22:42 GMT+02:00 Hackety Man :

> *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, 99) AS
> int_distinct, 'Test'::text || generate_series(0, 99)::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('Test501');
> EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
> LOWER(text_distinct) = LOWER('Test501');
>
> 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) = 'test501'::text)
>
>  Rows Removed by Filter: 33
>
>  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) = 'test501'::text)
>
>   Rows Removed by Filter: 33
>
>   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('
> Test501');
>
> 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

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

2018-04-16 Thread Hackety Man

*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, 99) AS
int_distinct, 'Test'::text || generate_series(0, 99)::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('Test501');
EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test501');

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) =
'test501'::text)

 Rows Removed by Filter:
33

 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) =
'test501'::text)

  Rows Removed by Filter:
33

  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('Test501');

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) = 'test501'::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

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

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

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

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

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

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

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

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