bug or lacking doc hint

On 6/25/23 10:01, Marc Millas wrote:

Hi,

I have had a perf (++) pb with a join plan  cf the pb with join plan thread.
I did simplify the thing up to when its a simple join between a 15M lines
table and a 30k lines table.
if I put in the on part something like table1.a=table2.b, Postgres does
the job in around 1 seconde.

if in the on part of the join I write table1.a=table2.b OR
substr(table1.c,x,x) =table2.d

That left side SUBSTR() can't be good for the query optimizer.  (Or I'm old
and still using habits picked up 25 years ago on a different RDBMS.)

Maybe a function index on table1 would help.

--
Born in Arizona, moved to Babylonia.

On Mon, 26 Jun 2023 at 03:02, Marc Millas <marc.millas@mokadb.com> wrote:

When I ask this list, David Rowley suggest to rewrite the SQL, replacing the OR by a union.

Fine, this do work, even if a bit complex as the original SQL was a set of intricate joins.

So, either this behaviour ( postgres unable to find an appropriate plan for join with OR clauses)
is a true bug which is waiting to be corrected, either the doc is lacking a chapter on which one can read that Postgres does NOT support that syntax as soon as there is some data volumes.

The problem is that out of the 3 methods PostgreSQL uses to join
tables, only 1 of them supports join conditions with an OR clause.
Merge Join cannot do this because results can only be ordered one way
at a time. Hash Join technically could do this, but it would require
that it built multiple hash tables. Currently, it only builds one
table. That leaves Nested Loop as the join method to implement joins
with OR clauses. Unfortunately, nested loops are quadratic and the
join condition must be evaluated once per each cartesian product row.
That does not scale well.

Tom Lane did start some work [1]/messages/by-id/20180904015910.GA1797012@rfd.leadboat.com to allow the planner to convert some
queries to use UNION instead of evaluating OR clauses, but, if I
remember correctly, it didn't handle ORs in join conditions, though
perhaps having it do that would be a natural phase 2. I don't recall
why the work stopped.

So, now I am writing some kind of recipe book for the users of that DB.

What should I write ?

You're asking a public mailing list to write private documentation for
the company you work for? hmm.

David

[1]: /messages/by-id/20180904015910.GA1797012@rfd.leadboat.com

On Sun, Jun 25, 2023 at 9:35 PM David Rowley <dgrowleyml@gmail.com> wrote:

On Mon, 26 Jun 2023 at 03:02, Marc Millas <marc.millas@mokadb.com> wrote:

When I ask this list, David Rowley suggest to rewrite the SQL, replacing

the OR by a union.

Fine, this do work, even if a bit complex as the original SQL was a set

of intricate joins.

So, either this behaviour ( postgres unable to find an appropriate plan

for join with OR clauses)

is a true bug which is waiting to be corrected, either the doc is

lacking a chapter on which one can read that Postgres does NOT support that
syntax as soon as there is some data volumes.

The problem is that out of the 3 methods PostgreSQL uses to join
tables, only 1 of them supports join conditions with an OR clause.
Merge Join cannot do this because results can only be ordered one way
at a time. Hash Join technically could do this, but it would require
that it built multiple hash tables. Currently, it only builds one
table. That leaves Nested Loop as the join method to implement joins
with OR clauses. Unfortunately, nested loops are quadratic and the
join condition must be evaluated once per each cartesian product row.
That does not scale well.

Tom Lane did start some work [1] to allow the planner to convert some
queries to use UNION instead of evaluating OR clauses, but, if I
remember correctly, it didn't handle ORs in join conditions, though
perhaps having it do that would be a natural phase 2. I don't recall
why the work stopped.

So, now I am writing some kind of recipe book for the users of that DB.

What should I write ?

You're asking a public mailing list to write private documentation for
the company you work for? hmm.

looks like some kind of misunderstanding:
what I am asking is: should I , or am I allowed to write that, according to
the community, Postgres is unable to do something ?

David

[1]
/messages/by-id/20180904015910.GA1797012@rfd.leadboat.com

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com

David Rowley <dgrowleyml@gmail.com> writes:

The problem is that out of the 3 methods PostgreSQL uses to join
tables, only 1 of them supports join conditions with an OR clause.
Merge Join cannot do this because results can only be ordered one way
at a time. Hash Join technically could do this, but it would require
that it built multiple hash tables. Currently, it only builds one
table. That leaves Nested Loop as the join method to implement joins
with OR clauses. Unfortunately, nested loops are quadratic and the
join condition must be evaluated once per each cartesian product row.

We can do better than that if the OR'd conditions are each amenable
to an index scan on one of the tables: then it can be a nestloop with
a bitmap-OR'd inner index scan. I thought the upthread advice to
convert the substr() condition into something that could be indexed
was on-point.

Tom Lane did start some work [1] to allow the planner to convert some
queries to use UNION instead of evaluating OR clauses, but, if I
remember correctly, it didn't handle ORs in join conditions, though
perhaps having it do that would be a natural phase 2. I don't recall
why the work stopped.

As I recall, I was having difficulty convincing myself that
de-duplication of results (for cases where the same row satisfies
more than one of the OR'd conditions) would work correctly.
You can't just blindly make it a UNION because that might remove
identical rows that *should* appear more than once in the result.

regards, tom lane

On Sun, Jun 25, 2023 at 11:48 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

David Rowley <dgrowleyml@gmail.com> writes:

The problem is that out of the 3 methods PostgreSQL uses to join
tables, only 1 of them supports join conditions with an OR clause.
Merge Join cannot do this because results can only be ordered one way
at a time. Hash Join technically could do this, but it would require
that it built multiple hash tables. Currently, it only builds one
table. That leaves Nested Loop as the join method to implement joins
with OR clauses. Unfortunately, nested loops are quadratic and the
join condition must be evaluated once per each cartesian product row.

We can do better than that if the OR'd conditions are each amenable
to an index scan on one of the tables: then it can be a nestloop with
a bitmap-OR'd inner index scan. I thought the upthread advice to
convert the substr() condition into something that could be indexed
was on-point.

ok. but one of the tables within the join(s) tables is 10 billions rows,
splitted in 120 partitions. Creating something like 20 more indexes to
fulfill that condition do have its own problems.

Tom Lane did start some work [1] to allow the planner to convert some
queries to use UNION instead of evaluating OR clauses, but, if I
remember correctly, it didn't handle ORs in join conditions, though
perhaps having it do that would be a natural phase 2. I don't recall
why the work stopped.

As I recall, I was having difficulty convincing myself that
de-duplication of results (for cases where the same row satisfies
more than one of the OR'd conditions) would work correctly.
You can't just blindly make it a UNION because that might remove
identical rows that *should* appear more than once in the result.

I did rewrite the query using a cte and union(s). For that query, no dedup
point.
But my pb is that that DB will be used by a bunch of people writing raw
SQL queries, and I cannot let them write queries that are going to go on
for ages, and eventually crash over temp_file_limit after hours every now
and then.
So, my understanding of the above is that I must inform the users NOT to
use OR clauses into joins.
which maybe a pb by itself.
regards
Marc

regards, tom lane

Marc MILLAS

Sounds like the problem you are having is, the server is running out of
temporary resources for the operation that users are trying to do. So
according to Tom, on the postgres side, the operation cannot be optimized
further.

I think you have few choices here,
- See if increasing the resources of the server will allow them to run
the operation
- Ask users not to do that operation
- Use a extension like citus to scale horizontally

But I'm thinking why a massively inefficient join is needed in the first
place. Shouldn't joins be for following keyed relationships. So ideally a
unique indexed column, but at the very least an indexed column. Why is a
join required on a dynamically calculated substring? Can it be made into a
static computed value and indexed? Substring sounds like an op that should
be in the filter stage.

Can you describe your data model? Maybe we can give some specific advice.

Regards,
Avin

On Mon, Jun 26, 2023 at 3:57 AM Marc Millas <marc.millas@mokadb.com> wrote:

On Mon, Jun 26, 2023 at 5:47 AM Avin Kavish <avin@baseboard.ai> wrote:

Sounds like the problem you are having is, the server is running out of
temporary resources for the operation that users are trying to do. So
according to Tom, on the postgres side, the operation cannot be optimized
further.

I think you have few choices here,
- See if increasing the resources of the server will allow them to run
the operation
- Ask users not to do that operation
- Use a extension like citus to scale horizontally

But I'm thinking why a massively inefficient join is needed in the first
place. Shouldn't joins be for following keyed relationships. So ideally a
unique indexed column, but at the very least an indexed column. Why is a
join required on a dynamically calculated substring? Can it be made into a
static computed value and indexed? Substring sounds like an op that should
be in the filter stage.

Can you describe your data model? Maybe we can give some specific advice.

There is a set of big tables containing fine grain health data. The DB is
accessed by people doing research on various kind of sicknesses. So, by
nature, all columns (I mean ALL) can be used for every kind of SQL
including, obviously, lots of intricated joins.
From time to time, People write requests that may take more than 2 days to
complete.
So the idea of indexing 'appropriate' columns translate in indexing all
columns, of all tables, including the big ones with 30+ columns.
with only main keys indexes, the DB is already 15TB+.
and my own experience of putting 30 indexes on one table is not very
positive.
so...
BTW rewriting the original request using cte and union does complete in 134
seconds, doing 3 merge join.

And I have one more question: the explain analyze plan shows that Postgres
decided to do external sorts using around 2 GB of disk space.
I did a set work_mem to '4GB' to try to have those sorts in memory. No
effect. How can I tell the planner to do those sort in memory ?? thanks

On 6/26/23 07:22, Marc Millas wrote:

On Mon, Jun 26, 2023 at 5:47 AM Avin Kavish <avin@baseboard.ai> wrote:

Sounds like the problem you are having is, the server is running out
of temporary resources for the operation that users are trying to do.
So according to Tom, on the postgres side, the operation cannot be
optimized further.

I think you have few choices here,
  - See if increasing the resources of the server will allow them to
run the operation
  - Ask users not to do that operation
  - Use a extension like citus to scale horizontally

But I'm thinking why a massively inefficient join is needed in the
first place. Shouldn't joins be for following keyed relationships. So
ideally a unique indexed column, but at the very least an indexed
column. Why is a join required on a dynamically calculated substring?
Can it be made into a static computed value and indexed? Substring
sounds like an op that should be in the filter stage.

Can you describe your data model? Maybe we can give some specific advice.

There is a set of big tables containing fine grain health data. The DB is
accessed by people doing research on various kind of sicknesses. So, by
nature, all columns (I mean ALL) can be used for every kind of SQL
including, obviously, lots of intricated joins.

This sounds like the kind of problem normally solved by data warehouses.  Is
your schema designed like a DW, or is it in 3NF?

--
Born in Arizona, moved to Babylonia.

On Mon, Jun 26, 2023 at 4:05 PM Ron <ronljohnsonjr@gmail.com> wrote:

On 6/26/23 07:22, Marc Millas wrote:

On Mon, Jun 26, 2023 at 5:47 AM Avin Kavish <avin@baseboard.ai> wrote:

Sounds like the problem you are having is, the server is running out of
temporary resources for the operation that users are trying to do. So
according to Tom, on the postgres side, the operation cannot be optimized
further.

I think you have few choices here,
- See if increasing the resources of the server will allow them to run
the operation
- Ask users not to do that operation
- Use a extension like citus to scale horizontally

But I'm thinking why a massively inefficient join is needed in the first
place. Shouldn't joins be for following keyed relationships. So ideally a
unique indexed column, but at the very least an indexed column. Why is a
join required on a dynamically calculated substring? Can it be made into a
static computed value and indexed? Substring sounds like an op that should
be in the filter stage.

Can you describe your data model? Maybe we can give some specific advice.

There is a set of big tables containing fine grain health data. The DB is
accessed by people doing research on various kind of sicknesses. So, by
nature, all columns (I mean ALL) can be used for every kind of SQL
including, obviously, lots of intricated joins.

This sounds like the kind of problem normally solved by data warehouses.
Is your schema designed like a DW, or is it in 3NF?

it's, indeed, some kind of dwh.
but it's neither a star nor a snowflake .at least not used like those
standard schemas.
in one of the big tables (10 billions+ rows), there is around 60 columns,
describing one event: some guy have had a given sickness, got a given medoc
etc
The pb is that its not one simple event with a set of dimensions,
the people using that db are NOT looking for an event according to
various criterias, they are looking for correlations between each of the
60+ columns.
As a consequence very few indexes are used as most requests end in some
kind of huge sequential reads.
The machine was built for this and perform well, but some requests are
posing pb and we must find solutions/workaround.
one of the users did rewrite the request using a select distinct matched
with left join(s) and table.a is not null set of conditions.
looks crazy, but does work. I'll get the request tomorrow.

--
Born in Arizona, moved to Babylonia.

Marc MILLAS

On Mon, Jun 26, 2023 at 4:21 PM Marc Millas <marc.millas@mokadb.com> wrote:

On Mon, Jun 26, 2023 at 4:05 PM Ron <ronljohnsonjr@gmail.com> wrote:

On 6/26/23 07:22, Marc Millas wrote:

On Mon, Jun 26, 2023 at 5:47 AM Avin Kavish <avin@baseboard.ai> wrote:

Sounds like the problem you are having is, the server is running out of
temporary resources for the operation that users are trying to do. So
according to Tom, on the postgres side, the operation cannot be optimized
further.

I think you have few choices here,
- See if increasing the resources of the server will allow them to run
the operation
- Ask users not to do that operation
- Use a extension like citus to scale horizontally

But I'm thinking why a massively inefficient join is needed in the first
place. Shouldn't joins be for following keyed relationships. So ideally a
unique indexed column, but at the very least an indexed column. Why is a
join required on a dynamically calculated substring? Can it be made into a
static computed value and indexed? Substring sounds like an op that should
be in the filter stage.

Can you describe your data model? Maybe we can give some specific advice.

There is a set of big tables containing fine grain health data. The DB is
accessed by people doing research on various kind of sicknesses. So, by
nature, all columns (I mean ALL) can be used for every kind of SQL
including, obviously, lots of intricated joins.

This sounds like the kind of problem normally solved by data warehouses.
Is your schema designed like a DW, or is it in 3NF?

it's, indeed, some kind of dwh.
but it's neither a star nor a snowflake .at least not used like those
standard schemas.
in one of the big tables (10 billions+ rows), there is around 60 columns,
describing one event: some guy have had a given sickness, got a given medoc
etc
The pb is that its not one simple event with a set of dimensions,
the people using that db are NOT looking for an event according to
various criterias, they are looking for correlations between each of the
60+ columns.
As a consequence very few indexes are used as most requests end in some
kind of huge sequential reads.
The machine was built for this and perform well, but some requests are
posing pb and we must find solutions/workaround.
one of the users did rewrite the request using a select distinct matched
with left join(s) and table.a is not null set of conditions.
looks crazy, but does work. I'll get the request tomorrow.

Marc,
Something we did for cases like this... We actually created views that
handled the complex joining.
Then we trained users to select from the views (as opposed to the tables).

The upside of this approach is that you can really optimize the views for
the joins. And then the results get filtered by
the where clauses they provide.

Usually a DBA or heavy DB user creates the views, and gets the permission
to create the indexes that speed them up.

The DOWNSIDE of this approach is that if you are constantly adding
columns, you have to recreate your views.
And please be careful with Views that depend on views. While it works, it
creates downsides when you attempt to
change a view. Often having to drop all of the downstream views, and
recreate them.

HTH

Kirk

Hi All

Got a weird one. I am using the regex below to case match numeric only values.

'^([0-9]+[.]?[0-9]*)$'

This works well by and large but not for a value like “1234:567”. This seems to match positive and then fails due to not being an actual number in the subsequent cast.

Any ideas? (PG13)

Z

On Tue, Jun 27, 2023 at 11:59 AM Zahir Lalani <ZahirLalani@oliver.agency>
wrote:

Hi All

Got a weird one. I am using the regex below to case match numeric only
values.

'^([0-9]+[.]?[0-9]*)$'

This works well by and large but not for a value like “1234:567”. This
seems to match positive and then fails due to not being an actual number in
the subsequent cast.

Works (returns false) here:
select '1234:567' ~ '^([0-9]+[.]?[0-9]*)$';

https://mail.google.com/mail/u/0/?tab=mm&amp;zx=y3hfqt48pyg7#all/FMfcgzGsnLNNXcRVCVNpjQhGknMSVLKn

David J.

Probably not much help but it seems to work ok for me (unless I'm doing
something stupid). You didn't actually say how/where you were doing the
regex match, but the '~' operator seems to work ok.

[~/git/caas-sqs-consumer] psql
psql (15.3 (Homebrew), server 13.7)
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384,
compression: off)
Type "help" for help.

bcaas=> select '1234:56' ~ '^([0-9]+[.]?[0-9]*)$';
?column?
----------
f
(1 row)

bcaas=> select '1234.56' ~ '^([0-9]+[.]?[0-9]*)$';
?column?
----------
t
(1 row)

On Wed, Jun 28, 2023 at 4:59 AM Zahir Lalani <ZahirLalani@oliver.agency>
wrote:

On Tue, 27 Jun 2023 at 14:59, Zahir Lalani <ZahirLalani@oliver.agency> wrote:

Hi All

Got a weird one. I am using the regex below to case match numeric only values.

'^([0-9]+[.]?[0-9]*)$'

This works well by and large but not for a value like “1234:567”. This seems to match positive and then fails due to not being an actual number in the subsequent cast.

Any ideas? (PG13)

Z

When executed from psql it works ok. What tool did you use to run your
query? I would suspect that the tool does interpret square brackets
somehow and your regex actually becomes '^([0-9]+.?[0-9]*)$' which
matches any character between digits.

If you enable query logging in your system - either uncomment
log_statement = 'all' in postgresql.conf or run ALTER SYSTEM SET
log_statement = 'all'; and restart, then you will find the actual
query in server's log file:
2023-06-27 16:12:35.221 EDT [23580] LOG: statement: select '1234:56'
~ '^([0-9]+[.]?[0-9]*)$';

make sure it remains the same.
--
Sergey

-----Original Message-----

Hi All

Got a weird one. I am using the regex below to case match numeric only

values.

'^([0-9]+[.]?[0-9]*)$'

This works well by and large but not for a value like “1234:567”. This seems

to match positive and then fails due to not being an actual number in the
subsequent cast.

Any ideas? (PG13)

Z

My apologies for wasting your time ☹

The issue was in an unrelated field!!

Thank you

Z