Обсуждение: Classes and Inheritance
<i>I didn't know if this had made it to the list or not... if it has, please excuse...<br /><br /></i>I've been trying
towork on a system of using classes as a variation on normalization of data.<br /> However, I've run into a really
frustratingproblem. I had posted the following but not in this form to the PGSQL SQL list.<br /><br /> From that
posting...<br/> --begin-- <dl><dd>create table foo (id int8); <dd>create table bar1 (name text) inherits (foo);
<dd>createtable bar2 (data text) inherits (foo); <dd>create table hybrid ( ) inherits (bar1, bar2);<br /><br /><b>
</b><dd>INSERTINTO foo VALUES (1); <dd>INSERT INTO bar1 VALUES (2,'myname'); <dd>INSERT INTO bar2 VALUES (3,'mydata');
<dd>INSERTINTO hybrid VALUES (4,'morename','moredata');<br /><br /><br /><br /><br /><br /><dd>I want to do a <b>SELECT
*FROM foo*</b>; but I only get the 'id' column as in :<br /><br /><font face="Courier, Courier"> </font><dd>id <dd>---
<dd> 1<dd> 2 <dd> 3 <dd> 4<br /><br /><br /><br /><br /><br /><dd>What would be the query to get the following table or
amagical way to expand children without knowing the children's table names?<br /><br /><dd>I had originally hoped that
<b>SELECT* FROM foo*</b> would yield the following, but it's not so.<br /><br /><font face="Courier, Courier">
</font><dd>id| name | data <dd>---+------------+------------- <dd> 1 | null | null <dd> 2 | 'myname' |
null<dd> 3 | null | 'mydata' <dd> 4 | 'morename' | 'moredata' </dl>--end--<br /><br /> I can get the same result
bydoing the following query...<br /><br /><b>SELECT * FROM hybrid UNION \<br /> (SELECT id,null,data FROM bar2)
UNION\<br /> (SELECT id,name,null FROM bar1) UNION \<br /> (SELECT id,null,null FROM foo)<br /><br
/></b>However,this doesn't allow for flexibility in the declaration of classes as all queries have to be rewritten.<br
/><br/><b>Proposal<br /><br /></b>What I would like to suggest although it may be too much or a little odd is to have a
wayfor the column listing to be expanded based on the children. So far, I have been unable to construct a query or
functionthat would run through and expand all columns based on class/table name and its' children and return a suitable
union.<br/><br /> What if following a class/table by + did this.<br /><br /> For example the above query could be
producedby the following statement:<br /><b> SELECT * FROM foo+<br /><br /></b>The pattern I noticed it works
beststarting from the farthest child(ren) since the mininum number of columns in a child is >= the number of columns
inthe parent. However, I also observed this could be really awkward depending on the number of columns ancestors had
inaddition to the depth of the tree. In the example give below, the table "hybrid" was unique in that it inherited
allfrom a common set of parents. It would be different if the farthest branches merged or if a branch contained a
differentdata type in the same column namespace.<br /><br /> I did figure a methodology on solving it:<br /><br /> 1.
getthe relid of the table/class;<br /> 2. using pg_inherits, resolve all dependent children.<br /> 3. create an array
ofall columns (may be data type conflicts so either resolve or error) {you could avoid conflicts with careful planning
onnames}<br /> 4. union all tables inserting nulls in non-available columns places.<br /><br /> Note: Step 2 might be
easilyfacilitated if an array of immediate children was available from pg_inherits table not just the parent, but then
insertingclasses might be expensive. It's a thought...<br /><br /> This has been very frustrating even in trying to get
informationfrom the pg_* system tables. And, ideally, a user should never have to look at those although it is nice
tosuch as in this case.<br /><br /> I'm afraid this all sounds like a complaint. Quite to the contrary, I've been more
thanpleased with postgresql and what it has to offer and continually look forward to newer releases, new features and
improvements.<br/><br /> Thomas<br /><br /> - <br /> - <b><u>Thomas Swan</u></b> <br
/>- Graduate Student - Computer Science<br /> - The University of Mississippi<br /> - <br /> - "People can be
categorizedinto two fundamental <br /> - groups, those that divide people into two groups <br /> - and those that
don't."
Hi,
This is something that has been discussed extensively in the hackers
list in recent times (mostly by me :). It is a reasonably extensive
change, affecting front and back ends etc. The syntax suggested has been
something like "select ** from foo" or "select % from foo". The status
of it is I have done the changes to libpq, and am looking into the
backend situation. If you want to help out it would certainly be
welcome.
Thomas Swan wrote:
>
> I didn't know if this had made it to the list or not... if it has,
> please excuse...
>
> I've been trying to work on a system of using classes as a variation
> on normalization of data.
> However, I've run into a really frustrating problem. I had posted the
> following but not in this form to the PGSQL SQL list.
>
> From that posting...
> --begin--
>
> create table foo (id int8);
> create table bar1 (name text) inherits (foo);
> create table bar2 (data text) inherits (foo);
> create table hybrid ( ) inherits (bar1, bar2);
>
> INSERT INTO foo VALUES (1);
> INSERT INTO bar1 VALUES (2,'myname');
> INSERT INTO bar2 VALUES (3,'mydata');
> INSERT INTO hybrid VALUES (4,'morename','moredata');
>
> I want to do a SELECT * FROM foo*; but I only get the 'id' column
> as in :
>
> id
> ---
> 1
> 2
> 3
> 4
>
> What would be the query to get the following table or a magical
> way to expand children without knowing the children's table
> names?
>
> I had originally hoped that SELECT * FROM foo* would yield the
> following, but it's not so.
>
> id | name | data
> ---+------------+-------------
> 1 | null | null
> 2 | 'myname' | null
> 3 | null | 'mydata'
> 4 | 'morename' | 'moredata'
>
> --end--
>
> I can get the same result by doing the following query...
>
> SELECT * FROM hybrid UNION \
> (SELECT id,null,data FROM bar2) UNION \
> (SELECT id,name,null FROM bar1) UNION \
> (SELECT id,null,null FROM foo)
>
> However, this doesn't allow for flexibility in the declaration of
> classes as all queries have to be rewritten.
>
> Proposal
>
> What I would like to suggest although it may be too much or a little
> odd is to have a way for the column listing to be expanded based on
> the children. So far, I have been unable to construct a query or
> function that would run through and expand all columns based on
> class/table name and its' children and return a suitable union.
>
> What if following a class/table by + did this.
>
> For example the above query could be produced by the following
> statement:
> SELECT * FROM foo+
>
> The pattern I noticed it works best starting from the farthest
> child(ren) since the mininum number of columns in a child is >= the
> number of columns in the parent. However, I also observed this could
> be really awkward depending on the number of columns ancestors had in
> addition to the depth of the tree. In the example give below, the
> table "hybrid" was unique in that it inherited all from a common set
> of parents. It would be different if the farthest branches merged or
> if a branch contained a different data type in the same column
> namespace.
>
> I did figure a methodology on solving it:
>
> 1. get the relid of the table/class;
> 2. using pg_inherits, resolve all dependent children.
> 3. create an array of all columns (may be data type conflicts so
> either resolve or error) {you could avoid conflicts with careful
> planning on names}
> 4. union all tables inserting nulls in non-available columns places.
>
> Note: Step 2 might be easily facilitated if an array of immediate
> children was available from pg_inherits table not just the parent, but
> then inserting classes might be expensive. It's a thought...
>
> This has been very frustrating even in trying to get information from
> the pg_* system tables. And, ideally, a user should never have to
> look at those although it is nice to such as in this case.
>
> I'm afraid this all sounds like a complaint. Quite to the contrary,
> I've been more than pleased with postgresql and what it has to offer
> and continually look forward to newer releases, new features and
> improvements.
>
> Thomas
>
> -
> - Thomas Swan
> - Graduate Student - Computer Science
> - The University of Mississippi
> -
> - "People can be categorized into two fundamental
> - groups, those that divide people into two groups
> - and those that don't."