Migrating an application with Oracle temporary tables

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Migrating an application with Oracle temporary tables

Mark Zellers

One of the issues I have run into in migrating from Oracle to PostgreSQL is the difference in temporary table behavior.  I've come up with what I think is a cunning plan, but I wonder if anyone with more PostgreSQL understanding can shoot holes in it.

 

Oracle defines their temporary tables statically, such that the first use in a session instantiates a private instance of the table.  That implies that you need to jump through flaming hoops in order to be able to use temporary tables in stored procedures and functions.

 

My idea was to create a "prototype" table (which will never have any rows in it) and, when I need to use the temporary table, create the temporary table such that it hides the prototype.  I have a simple example that seems to work, although before I invest more effort in using it for real work, I'd like to get a second opinion as to whether this approach is going to fail in a more complex scenario.

 

 

drop function if exists bar;

drop table if exists foo;

 

create table FOO(X INTEGER not null constraint PTCC_FOO check (X is NULL));

commit;

 

create function bar() returns INTEGER

as

$$

DECLARE

   result INTEGER;

begin 

    select SUM(X) into result from FOO ;

    return result;

end;

$$ language plpgsql;

 

commit;

 

create temporary table FOO(X INTEGER) on commit drop;

alter table FOO add constraint PK_FOO primary KEY(X);

insert into FOO(X) VALUES(1);

insert into FOO(X) VALUES(2);

insert into FOO(X) VALUES(3);

 

select 'Expect to see 6' as Expectation, BAR();

commit;

 

 

create temporary table FOO(X INTEGER) on commit drop;

alter table FOO add constraint PK_FOO primary key (X);

insert into FOO(X) VALUES(1);

insert into FOO(X) VALUES(2);

insert into FOO(X) VALUES(3);

insert into FOO(X) VALUES(4);

select 'Expect to see 10' as Expectation, BAR();

rollback;

 

create temporary table FOO(X INTEGER) on commit drop;

insert into FOO(X) VALUES(1);

insert into FOO(X) VALUES(2);

select 'Expect to see 3' as Expectation, BAR();

commit;

 

 

What I am not clear on is what the rules are as to when a function/procedure is effectively recompiled.  Is there a danger that. assuming the temporary table is created for a session that one session might see another session's data due to the procedure having effectively compiled the temporary table into its definition?


While this approach does have the disadvantage of requiring the application to define the temporary table before using it (which could be as simple as using `CREATE TABLE AS SELECT * FROM prototype_table`), it seems simpler and potentially more performant than the approach I found here: https://www.codeproject.com/Articles/1176045/Oracle-style-global-temporary-tables-for-PostgreSQ

It is also in direct opposition to this post I found: https://www.cybertec-postgresql.com/en/using-temporary-tables-the-way-they-should-not-be-used/

So far, I have not found a case where, as long as I don’t read or write to the permanent table, I get the wrong results from the above approach.  It allows me to minimize the impact on my application (basically, it means that at the start of any transaction that might need a certain temporary table, I need to manually create it.  The number of places I would need to do that is relatively finite, so I’m willing to take that hit, in exchange for not having to use dynamic SQL to refer to my temporary tables.


 

 

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Re: Migrating an application with Oracle temporary tables

Laurenz Albe
Mark Zellers wrote:

> One of the issues I have run into in migrating from Oracle to PostgreSQL is the difference in temporary table behavior.
> I've come up with what I think is a cunning plan, but I wonder if anyone with more PostgreSQL understanding can shoot holes in it.
>  
> Oracle defines their temporary tables statically, such that the first use in a session instantiates a private instance of the table.
> That implies that you need to jump through flaming hoops in order to be able to use temporary tables in stored procedures and functions.
>  
> My idea was to create a "prototype" table (which will never have any rows in it) and, when I need to use the temporary table,
> create the temporary table such that it hides the prototype.  I have a simple example that seems to work,
> although before I invest more effort in using it for real work, I'd like to get a second opinion as to whether this
> approach is going to fail in a more complex scenario.
[...]
> What I am not clear on is what the rules are as to when a function/procedure is effectively recompiled.
> Is there a danger that. assuming the temporary table is created for a session that one session might see another session's data due
> to the procedure having effectively compiled the temporary table into its definition?

There is no danger of that, because a function is parsed whenever it is executed, and certainly
different database sessions never share any "compiled version" of the function or query execution plans.

What does get cached are the execution plans of SQL statements in a PL/pgSQL function, but only
across different calls in the same database session.

This does not seem to cause problems in your case (the plans are probably invalidated; I am not certain),
but you can avoid that behavior by using dynamic SQL (EXECUTE 'SELECT ...').

> While this approach does have the disadvantage of requiring the application to define the temporary table before using it (which
> could be as simple as using `CREATE TABLE AS SELECT * FROM prototype_table`), it seems simpler and potentially more performant than
> the approach I found here: https://www.codeproject.com/Articles/1176045/Oracle-style-global-temporary-tables-for-PostgreSQ
>
> It is also in direct opposition to this post I found: https://www.cybertec-postgresql.com/en/using-temporary-tables-the-way-they-should-not-be-used/ 

In this case, the "trouble and pain" behavior is what you explicitly want, so don't worry.

> So far, I have not found a case where, as long as I don’t read or write to the permanent table, I get the wrong results from
> the above approach.  It allows me to minimize the impact on my application (basically, it means that at the start of any transaction
> that might need a certain temporary table, I need to manually create it.  The number of places I would need to do that is relatively finite,
> so I’m willing to take that hit, in exchange for not having to use dynamic SQL to refer to my temporary tables.

The biggest problem I see with your approach is that dropping temporary tables causes a DELETE
in the system catalogs "pg_class" and "pg_attribute", and if you do that very often, particularly
the latter table tends to get bloated.  Either don't drop temporary tables all the time or
make sure that "pg_attribute" gets vacuumed fast enough.

Other than that, I don't really see the need for keeping a permanent "blueprint" table
around; all this can lead to is confusion.  True, it is cute to use CREATE TABLE ... (LIKE ...),
but a few lines more in your code won't kill you.  Particularly since you have that only once
in your code, right?

Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com



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Re: Migrating an application with Oracle temporary tables

Pavel Stehule
In reply to this post by Mark Zellers
Hi

What I am not clear on is what the rules are as to when a function/procedure is effectively recompiled.  Is there a danger that. assuming the temporary table is created for a session that one session might see another session's data due to the procedure having effectively compiled the temporary table into its definition?


it's not a problem - the plpgsql engine is implemented significantly differently than PL/SQL

plpgsql code is not compiled - it is just validated in "create" time and it is stored in original text form. When function is started first time in session, then source code is reparsed again to syntax tree, and this tree is evaluated. Inner SQL queries are prepared.

When you create and drop table, then prepared statements are automatically replaned.

Sometimes can be strange (and difficult) so plpgsql is similar to PL/SQL, but the implementation is maximally different - so some knowledges and experience are portable (and lot of related to internals) are not portable.

Regards

Pavel



While this approach does have the disadvantage of requiring the application to define the temporary table before using it (which could be as simple as using `CREATE TABLE AS SELECT * FROM prototype_table`), it seems simpler and potentially more performant than the approach I found here: https://www.codeproject.com/Articles/1176045/Oracle-style-global-temporary-tables-for-PostgreSQ

It is also in direct opposition to this post I found: https://www.cybertec-postgresql.com/en/using-temporary-tables-the-way-they-should-not-be-used/

So far, I have not found a case where, as long as I don’t read or write to the permanent table, I get the wrong results from the above approach.  It allows me to minimize the impact on my application (basically, it means that at the start of any transaction that might need a certain temporary table, I need to manually create it.  The number of places I would need to do that is relatively finite, so I’m willing to take that hit, in exchange for not having to use dynamic SQL to refer to my temporary tables.


 

 

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RE: Migrating an application with Oracle temporary tables

Mark Zellers
In reply to this post by Laurenz Albe
Laurenz,

I thought I needed the prototype table to be able to define functions and procedures that refer to the temporary table but do not create it.

Perhaps my assumption that I need the table to exist (whether as a temporary table or as a permanent table) in order to define the function/procedure is incorrect.  I'll take a look at that.

I did find a scenario where this approach does run into trouble.  That is, if the function/procedure is executed against the permanent table and then you go to run it against a temporary table.  In that case, I do get the wrong answer, and I haven't yet figured out how to reset that without dropping the procedure and re-defining it.  For my purposes, that is "good enough" -- I can promise not to run such procedures against the temporary table.

Thanks for the reply,

Mark Z.




-----Original Message-----
From: Laurenz Albe <[hidden email]>
Sent: Thursday, May 2, 2019 1:00 AM
To: Mark Zellers <[hidden email]>; [hidden email]
Subject: Re: Migrating an application with Oracle temporary tables

Mark Zellers wrote:

> One of the issues I have run into in migrating from Oracle to PostgreSQL is the difference in temporary table behavior.
> I've come up with what I think is a cunning plan, but I wonder if anyone with more PostgreSQL understanding can shoot holes in it.
>  
> Oracle defines their temporary tables statically, such that the first use in a session instantiates a private instance of the table.
> That implies that you need to jump through flaming hoops in order to be able to use temporary tables in stored procedures and functions.
>  
> My idea was to create a "prototype" table (which will never have any
> rows in it) and, when I need to use the temporary table, create the
> temporary table such that it hides the prototype.  I have a simple
> example that seems to work, although before I invest more effort in using it for real work, I'd like to get a second opinion as to whether this approach is going to fail in a more complex scenario.
[...]
> What I am not clear on is what the rules are as to when a function/procedure is effectively recompiled.
> Is there a danger that. assuming the temporary table is created for a
> session that one session might see another session's data due to the procedure having effectively compiled the temporary table into its definition?

There is no danger of that, because a function is parsed whenever it is executed, and certainly different database sessions never share any "compiled version" of the function or query execution plans.

What does get cached are the execution plans of SQL statements in a PL/pgSQL function, but only across different calls in the same database session.

This does not seem to cause problems in your case (the plans are probably invalidated; I am not certain), but you can avoid that behavior by using dynamic SQL (EXECUTE 'SELECT ...').

> While this approach does have the disadvantage of requiring the
> application to define the temporary table before using it (which could
> be as simple as using `CREATE TABLE AS SELECT * FROM
> prototype_table`), it seems simpler and potentially more performant
> than the approach I found here:
> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.codeproject.c
> om_Articles_1176045_Oracle-2Dstyle-2Dglobal-2Dtemporary-2Dtables-2Dfor
> -2DPostgreSQ&d=DwIDaQ&c=DS6PUFBBr_KiLo7Sjt3ljp5jaW5k2i9ijVXllEdOozc&r=
> WK2G_319M5jcG-UdecAitIGR-UzTlASAsP7ybqXtD2Y&m=gYKOaSte_-ZRnU2RgctTQfAn
> J9BhFhxowJBy7upH7KE&s=I_x0KCEfHgBXOoSRYXpAehb5uRodwY11-m6JRYf9A_Y&e=
>
> It is also in direct opposition to this post I found:
> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.cybertec-2Dpo
> stgresql.com_en_using-2Dtemporary-2Dtables-2Dthe-2Dway-2Dthey-2Dshould
> -2Dnot-2Dbe-2Dused_&d=DwIDaQ&c=DS6PUFBBr_KiLo7Sjt3ljp5jaW5k2i9ijVXllEd
> Oozc&r=WK2G_319M5jcG-UdecAitIGR-UzTlASAsP7ybqXtD2Y&m=gYKOaSte_-ZRnU2Rg
> ctTQfAnJ9BhFhxowJBy7upH7KE&s=OJVABOenTvrP2J1ubWXbaMayh1AVLOp2ieQ4HeacV
> 0I&e=

In this case, the "trouble and pain" behavior is what you explicitly want, so don't worry.

> So far, I have not found a case where, as long as I don’t read or
> write to the permanent table, I get the wrong results from the above
> approach.  It allows me to minimize the impact on my application
> (basically, it means that at the start of any transaction that might need a certain temporary table, I need to manually create it.  The number of places I would need to do that is relatively finite, so I’m willing to take that hit, in exchange for not having to use dynamic SQL to refer to my temporary tables.

The biggest problem I see with your approach is that dropping temporary tables causes a DELETE in the system catalogs "pg_class" and "pg_attribute", and if you do that very often, particularly the latter table tends to get bloated.  Either don't drop temporary tables all the time or make sure that "pg_attribute" gets vacuumed fast enough.

Other than that, I don't really see the need for keeping a permanent "blueprint" table around; all this can lead to is confusion.  True, it is cute to use CREATE TABLE ... (LIKE ...), but a few lines more in your code won't kill you.  Particularly since you have that only once in your code, right?

Yours,
Laurenz Albe
--
Cybertec | https://urldefense.proofpoint.com/v2/url?u=https-3A__www.cybertec-2Dpostgresql.com&d=DwIDaQ&c=DS6PUFBBr_KiLo7Sjt3ljp5jaW5k2i9ijVXllEdOozc&r=WK2G_319M5jcG-UdecAitIGR-UzTlASAsP7ybqXtD2Y&m=gYKOaSte_-ZRnU2RgctTQfAnJ9BhFhxowJBy7upH7KE&s=Sg60ggj5ptVnAaaRbEl0NpGMBm8_GoHO_msgOqa8vgE&e=

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Re: Migrating an application with Oracle temporary tables

Laurenz Albe
On Thu, 2019-05-02 at 16:55 +0000, Mark Zellers wrote:
> I thought I needed the prototype table to be able to define functions and procedures that refer to the temporary table but do not create it.
>
> Perhaps my assumption that I need the table to exist (whether as a temporary table or as a permanent table) in
> order to define the function/procedure is incorrect.  I'll take a look at that.

You don't need the table to exist at function definition time.
The following works just fine, even if the table does not exist:

CREATE FUNCTION f() RETURNS void LANGUAGE plpgsql AS $$BEGIN PERFORM * FROM notexists; END;$$;

This is because functions are not parsed when they are defined.

> I did find a scenario where this approach does run into trouble.  That is, if the function/procedure is executed
> against the permanent table and then you go to run it against a temporary table.  In that case, I do get the
> wrong answer, and I haven't yet figured out how to reset that without dropping the procedure and re-defining it.
> For my purposes, that is "good enough" -- I can promise not to run such procedures against the temporary table.

Yes, that would cause a problem.

The SQL statement "DISCARD PLANS" should fix the problem.

Yours,
Laurenz Albe

--
Cybertec | https://www.cybertec-postgresql.com



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Re: Migrating an application with Oracle temporary tables

Pavel Stehule


pá 3. 5. 2019 v 8:19 odesílatel Laurenz Albe <[hidden email]> napsal:
On Thu, 2019-05-02 at 16:55 +0000, Mark Zellers wrote:
> I thought I needed the prototype table to be able to define functions and procedures that refer to the temporary table but do not create it.
>
> Perhaps my assumption that I need the table to exist (whether as a temporary table or as a permanent table) in
> order to define the function/procedure is incorrect.  I'll take a look at that.

You don't need the table to exist at function definition time.
The following works just fine, even if the table does not exist:

CREATE FUNCTION f() RETURNS void LANGUAGE plpgsql AS $$BEGIN PERFORM * FROM notexists; END;$$;

This is because functions are not parsed when they are defined.

It is not fully correct - function with queries are parsed and syntax check is done. But semantic check is  deferred on run time.

Regards

Pavel


> I did find a scenario where this approach does run into trouble.  That is, if the function/procedure is executed
> against the permanent table and then you go to run it against a temporary table.  In that case, I do get the
> wrong answer, and I haven't yet figured out how to reset that without dropping the procedure and re-defining it.
> For my purposes, that is "good enough" -- I can promise not to run such procedures against the temporary table.

Yes, that would cause a problem.

The SQL statement "DISCARD PLANS" should fix the problem.

Yours,
Laurenz Albe

--
Cybertec | https://www.cybertec-postgresql.com



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Re: Migrating an application with Oracle temporary tables

Michael Lewis
I did find a scenario where this approach does run into trouble.  That is, if the function/procedure is executed against the permanent table and then you go to run it against a temporary table.  In that case, I do get the wrong answer, and I haven't yet figured out how to reset that without dropping the procedure and re-defining it.  For my purposes, that is "good enough" -- I can promise not to run such procedures against the temporary table.

You may want to be explicit and refer to the table with schema like select * from public.table_name OR select * from pg_temp.table_name to avoid confusion caused by the default behavior of using the temp table if it exists.