Обсуждение: Mimic ALIAS in Postgresql?

On 1/16/24 09:20, Ron Johnson wrote:
> Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a 
> different name (while also referring to it by the original name).
> 
> We have an application running on DB2/UDB which (for reasons wholly 
> unknown to me, and probably also to the current developer) extensively 
> uses this with two schemas: MTUSER and MTQRY.  For example, sometimes 
> refer to MTUSER.sometable and other times refer to it as MYQRY.sometable.

Just to be clear the table name <sometable> is the same in each schema, 
correct?

In other words setting search_path would only help if was set per 
session depending on which schema.<sometable> you wanted to access.

> 
> My goal is to present a way to migrate from UDB to PG with as few 
> application changes as possible.  Thus, the need to mimic aliases.
> 
> Maybe updatable views?
> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;
> 

-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 1/16/24 09:20, Ron Johnson wrote:
> Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a 
> different name (while also referring to it by the original name).
> 

> 
> Maybe updatable views?
> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;
> 

Assuming sometable is the same name in both schemas then the above will 
not work as:

https://www.postgresql.org/docs/current/sql-createview.html

"The name of the view must be distinct from the name of any other
relation (table, sequence, index, view, materialized view, or foreign
table) in the same schema."

You would get a conflict with the existing table MTQRY.sometable.

Though I noticed you have both MTQRY and MYQRY referring to the same 
thing, I think.

-- 
Adrian Klaver
adrian.klaver@aklaver.com

On Tue, Jan 16, 2024 at 12:40 PM Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 1/16/24 09:20, Ron Johnson wrote:
> Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a
> different name (while also referring to it by the original name).
>

>
> Maybe updatable views?
> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;
>

Assuming sometable is the same name in both schemas then the above will
not work as:

https://www.postgresql.org/docs/current/sql-createview.html

"The name of the view must be distinct from the name of any other
relation (table, sequence, index, view, materialized view, or foreign
table) in the same schema."

You would get a conflict with the existing table MTQRY.sometable.

> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

 

But mtqry is not the same schema as mtuser..

dba=# create schema mtuser;
CREATE SCHEMA
dba=# create schema mtqry;
CREATE SCHEMA
dba=#
dba=# create table mtuser.sometable(f1 int);
CREATE TABLE
dba=#
dba=# create view mtqry.sometable as select * from mtuser.sometable;
CREATE VIEW

But what are the down-sides that I haven't thought of?

What happened to the MYQRY schema in your OP?

In the above you still have a relation with the same name in different schema.

How does that change the issue?

On Tue, Jan 16, 2024 at 2:24 PM Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 1/16/24 10:11 AM, Ron Johnson wrote:

On Tue, Jan 16, 2024 at 12:40 PM Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 1/16/24 09:20, Ron Johnson wrote:
> Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a
> different name (while also referring to it by the original name).
>

>
> Maybe updatable views?
> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;
>

Assuming sometable is the same name in both schemas then the above will
not work as:

https://www.postgresql.org/docs/current/sql-createview.html

"The name of the view must be distinct from the name of any other
relation (table, sequence, index, view, materialized view, or foreign
table) in the same schema."

You would get a conflict with the existing table MTQRY.sometable.

> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

 

But mtqry is not the same schema as mtuser..

dba=# create schema mtuser;
CREATE SCHEMA
dba=# create schema mtqry;
CREATE SCHEMA
dba=#
dba=# create table mtuser.sometable(f1 int);
CREATE TABLE
dba=#
dba=# create view mtqry.sometable as select * from mtuser.sometable;
CREATE VIEW

But what are the down-sides that I haven't thought of?

What happened to the MYQRY schema in your OP?

?

"... with two schemas: MTUSER and MTQRY. For example, sometimes refer to
MTUSER.sometable and other times refer to it as MYQRY.sometable.

"

Now if MYQRY.sometable in your OP is in fact MTQRY.sometable then this:

create view mtqry.sometable as select * from mtuser.sometable;

is not going to work as a view cannot have the same name as a table in the same schema.

On 1/16/24 11:20 AM, Ron Johnson wrote:
> Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a 
> different name (while also referring to it by the original name).
> 
> We have an application running on DB2/UDB which (for reasons wholly 
> unknown to me, and probably also to the current developer) extensively 
> uses this with two schemas: MTUSER and MTQRY.  For example, sometimes 
> refer to MTUSER.sometable and other times refer to it as MYQRY.sometable.
> 
> My goal is to present a way to migrate from UDB to PG with as few 
> application changes as possible.  Thus, the need to mimic aliases.
> 
> Maybe updatable views?
> CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

Based on the schema names one possibility is that the aliases are there 
as a pseudo-api between people/tools writing queries and the base 
tables. IE: if you needed to make a (maybe backwards-incompatible) 
change to "sometable" you now at least have the option of creating a 
MTQRY.sometable *view* that hides whatever change you're making to 
MTUSER.sometable.

In any case, yes, an updatable view would provide equivalent behavior in 
Postgres.
-- 
Jim Nasby, Data Architect, Austin TX

Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a different name (while also referring to it by the original name).

We have an application running on DB2/UDB which (for reasons wholly unknown to me, and probably also to the current developer) extensively uses this with two schemas: MTUSER and MTQRY.  For example, sometimes refer to MTUSER.sometable and other times refer to it as MYQRY.sometable.

My goal is to present a way to migrate from UDB to PG with as few application changes as possible.  Thus, the need to mimic aliases.

Maybe updatable views?

CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

On Tue, Jan 16, 2024 at 5:31 PM Rob Sargent <robjsargent@gmail.com> wrote:

On 1/16/24 10:20, Ron Johnson wrote:

Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a different name (while also referring to it by the original name).

We have an application running on DB2/UDB which (for reasons wholly unknown to me, and probably also to the current developer) extensively uses this with two schemas: MTUSER and MTQRY.  For example, sometimes refer to MTUSER.sometable and other times refer to it as MYQRY.sometable.

My goal is to present a way to migrate from UDB to PG with as few application changes as possible.  Thus, the need to mimic aliases.

Maybe updatable views?

CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

Isn't it time to get rid of that debt?  A sed -i 's/MTUSER/MTQRY/g' (or vice versa) ends what looks to me to be a split brain problem.  All the sql is in git right? :) 

Or perhaps you have to beef the sed up to use word boundaries just in case.

I'm not a Java web developer... 😁

On 1/16/24 4:57 PM, Rob Sargent wrote:
>>     Or perhaps you have to beef the sed up to use word boundaries just
>>     in case.
>>
>>
>> I'm not a Java web developer... 😁
> 
> You need to adjust you glasses if that's what you see me as.

Reality is that basically all modern (as in last 20 years) SQL access is 
via frameworks that all use their own language and come up with SQL 
based on that. How hard it'd be to bulk change the schema depends 
entirely on the framework.
-- 
Jim Nasby, Data Architect, Austin TX

On Tue, Jan 16, 2024 at 5:57 PM Rob Sargent <robjsargent@gmail.com> wrote:

On 1/16/24 15:39, Ron Johnson wrote:

On Tue, Jan 16, 2024 at 5:31 PM Rob Sargent <robjsargent@gmail.com> wrote:

On 1/16/24 10:20, Ron Johnson wrote:

Some RDBMSs have CREATE ALIAS, which allows you to refer to a table by a different name (while also referring to it by the original name).

We have an application running on DB2/UDB which (for reasons wholly unknown to me, and probably also to the current developer) extensively uses this with two schemas: MTUSER and MTQRY.  For example, sometimes refer to MTUSER.sometable and other times refer to it as MYQRY.sometable.

My goal is to present a way to migrate from UDB to PG with as few application changes as possible.  Thus, the need to mimic aliases.

Maybe updatable views?

CREATE VIEW mtqry.sometable AS SELECT * FROM mtuser.sometable;

Isn't it time to get rid of that debt?  A sed -i 's/MTUSER/MTQRY/g' (or vice versa) ends what looks to me to be a split brain problem.  All the sql is in git right? :) 

Or perhaps you have to beef the sed up to use word boundaries just in case.

I'm not a Java web developer... 😁

You need to adjust you glasses if that's what you see me as.

 

You're the one who apparently sees me as having any control over anything except when the backups run. 😞

On 1/16/24 4:57 PM, Rob Sargent wrote:

    Or perhaps you have to beef the sed up to use word boundaries just
    in case.


I'm not a Java web developer... 😁

You need to adjust you glasses if that's what you see me as.

Reality is that basically all modern (as in last 20 years) SQL access is via frameworks that all use their own language and come up with SQL based on that. How hard it'd be to bulk change the schema depends entirely on the framework.

On 1/16/24 6:41 PM, Rob Sargent wrote:
> On 1/16/24 17:39, Jim Nasby wrote:
>> On 1/16/24 4:57 PM, Rob Sargent wrote:
>>>>     Or perhaps you have to beef the sed up to use word boundaries just
>>>>     in case.
>>>>
>>>>
>>>> I'm not a Java web developer... 😁
>>>
>>> You need to adjust you glasses if that's what you see me as.
>>
>> Reality is that basically all modern (as in last 20 years) SQL access 
>> is via frameworks that all use their own language and come up with SQL 
>> based on that. How hard it'd be to bulk change the schema depends 
>> entirely on the framework.
> Hm, it's a string /somewhere/.  The rest of this thread might be accused 
> of adding to the problem.

No, it's not, at least not as a complete SQL statement. See [1] as an 
example of how this works in Ruby on Rails. Most modern frameworks work 
in a similar fashion: you DON'T write raw SQL, or anything that looks 
anything like it. In fact, many (most?) of these frameworks make it 
difficult to do anything in raw SQL because it completely breaks the 
paradigm of the framework.

Note that I'm talking about *frameworks*, not languages. But since most 
languages require huge amounts of boilerplate to create a web service or 
website it's not surprising that pretty much everyone uses frameworks. 
(Go is actually an interesting exception to this.)

1: https://guides.rubyonrails.org/active_record_querying.html#find
-- 
Jim Nasby, Data Architect, Austin TX

On 1/16/24 6:41 PM, Rob Sargent wrote:

On 1/16/24 17:39, Jim Nasby wrote:

On 1/16/24 4:57 PM, Rob Sargent wrote:

    Or perhaps you have to beef the sed up to use word boundaries just
    in case.


I'm not a Java web developer... 😁

You need to adjust you glasses if that's what you see me as.

Reality is that basically all modern (as in last 20 years) SQL access is via frameworks that all use their own language and come up with SQL based on that. How hard it'd be to bulk change the schema depends entirely on the framework.

Hm, it's a string /somewhere/.  The rest of this thread might be accused of adding to the problem.

No, it's not, at least not as a complete SQL statement. See [1] as an example of how this works in Ruby on Rails. Most modern frameworks work in a similar fashion: you DON'T write raw SQL, or anything that looks anything like it. In fact, many (most?) of these frameworks make it difficult to do anything in raw SQL because it completely breaks the paradigm of the framework.

Note that I'm talking about *frameworks*, not languages. But since most languages require huge amounts of boilerplate to create a web service or website it's not surprising that pretty much everyone uses frameworks. (Go is actually an interesting exception to this.)

1: https://guides.rubyonrails.org/active_record_querying.html#find