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Include e-mail exchange between Jan and Andreas to start info on rules.

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Thomas G. Lockhart 1998-04-28 14:52:46 +00:00
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doc/src/sgml/rules.sgml
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@ -7,7 +7,7 @@
them. Consequently, we will not attempt to explain the
actual syntax and operation of the <ProductName>Postgres</ProductName> rule system
here. Instead, you should read
<XRef LinkEnd="STON90b" EndTerm="[STON90b]"> to understand
[<XRef LinkEnd="STON90b" EndTerm="STON90b">] to understand
some of these points and the theoretical foundations of
the <ProductName>Postgres</ProductName> rule system before trying to use rules.
The discussion in this section is intended to provide
@ -20,9 +20,300 @@
is very powerful, and can be used for many things such
as query language procedures, views, and versions. The
power of this rule system is discussed in
<XRef LinkEnd="ONG90" EndTerm="[ONG90]">
[<XRef LinkEnd="ONG90" EndTerm="ONG90">]
as well as
<XRef LinkEnd="STON90b" EndTerm="[STON90b]">.
[<XRef LinkEnd="STON90b" EndTerm="STON90b">].
</Para>
<Sect1>
<Title>The Goodness of Rules</Title>
<Note>
<title>Editor's Note</title>
<Para>
This information resulted from an exchange of e-mail on 1998-02-20/22 between
<ULink url="mailto:jwieck@debis.com">Jan Wieck</ULink>,
<ULink url="mailto:Andreas.Zeugswetter@telecom.at">Andreas Zeugswetter</ULink>,
and
<ULink url="mailto:vadim@sable.krasnoyarsk.su">Vadim B. Mikheev</ULink>
on the subject.
</Para>
</Note>
<Para>
<ProgramListing>
From: Zeugswetter Andreas SARZ
To: Jan Wieck
</ProgramListing>
<Para>
Since we have so little documentation on the rules, I think we should save
every
little word describing them, so could you simply put the following into a
rules.readme
(undigested is still better than not adding it)
<Sect1>
<Title>Rewrite Rules versus Triggers</Title>
<Para>
> > Why I like the rewrite system is:
The benefits of the rewrite rules system include:
<VariableList>
<VarListEntry>
<Term>
Select Rewrite Possible
</Term>
<ListItem>
<Para>
A select trigger would be no good, due to optimizer concerns.
<Para>
Exactly that's what is done if you create a view. Postgres
creates a regular table (look at pg_class and into the
database directory) and then sets up a relation level instead
rewrite rule on select.
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
Dumb Client Possible
</Term>
<ListItem>
<Para>
The client can be really dumb, like MS Access or some other
standard ODBC tool
which does not know anything about funcs procs and the like
(even without using passthrough).
<Para>
The client must not know why and how and where the
data is left and coming from. But that's true in any case - a
trigger for each row on insert can do anything different and
push the data wherever it wants.
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
Rewrite rules are more powerful than views
</Term>
<ListItem>
<Para>
Views are only one special rule case in Postgres.
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
Optimizer Used
</Term>
<ListItem>
<Para>
It allows the optimizer to get involved (this is where triggers
fail per definition).
</ListItem>
</VarListEntry>
<VarListEntry>
<Term>
Simple to use
</Term>
<ListItem>
<Para>
Once understood it is very easy to use;
easier than triggers with <Acronym>C</Acronym> stored procedures at least.
</Para>
</ListItem>
</VarListEntry>
</VariableList>
<Para>
Optimizing again and again. If the rules aren't instead, the
querytree get's additional queries for every rule appended.
Have a table field that references an entry in another table
and this entry should have a refcount. So on update you must
decrease the refcount from the old ref and increase it on the
new. You create two rules so the UPDATE will result in 1
scan and 2 nestloops with scans inside - really optimized if
the referenced value doesn't change. And don't think that a
rule qual of NEW != CURRENT might help - that will result in
2 mergejoins where the scanned tuples are compared.
<Note>
<Para>
I fought that like a windmill, I guess it would be better to kill the
CURRENT keyword
with this meaning alltogether, since it only has the same meaning as the
tablename itself.
I have already crossed it out of my mind and don't miss anything.
</Para>
</Note>
I think there should instead be an OLD and NEW keyword
like in triggers:
<ProgramListing>
referencing old as <replaceable class="parameter">oldname</replaceable> new as <replaceable class="parameter">newname</replaceable>
</ProgramListing>
that only reference the tuples in memory.
<Para>
BTW, this sample doesn't work currently because the rules
queries are appended at the end of the querytree, thus the
decrement scan having the same qual will not find the old
tuple at all because it's already outdated
(command_counter_increment between processing the queries).
Referencing CURRENT in a rule is not what most people think
it is.
<Para>
The old 4.2 postgres had a second, instance level rule system
(prs2 stubs) that fired the rules actions when actually the
old tuple and the new projected tuple where handy. There you
could have made also things like 'UPDATE NEW SET a = 4' that
really modified the in memory tuple in the executors
expression context. Who the hell removed all that? It was so
nice :-(
<Note>
<Title>Editor's Note</Title>
<Para>
This feature was removed by Jolly et. al. prior to v1.0.x.
</Para>
</Note>
<Para>
Absolutely ! I did cry up when that was done, but nobody responded :-(
Well to be honest Vadim did respond with the trigger code, which made me
feel comfortable again.
<Para>
A really simple to write trigger can compare old != new and
only if send down the other two queries. This time they wont
be nestloops, they are simple scans. And the trigger can
arrange that the queries it uses are only parsed on it's
first of all calls and store the generated execution plans
permanently for quick execution (look at SPI_prepare).
<Para>
For the stored C procedures you're totally right. I don't
like the C functions because it requires postgres superuser
rights to develop them and thus I created PL/Tcl where joe
user can hack around without having complete access to the
whole database (look at src/pl/tcl). And someday after 6.3
release I'll really start on a plain PL/pgSQL implementation
that would give a normal user the opportunity to create
functions and triggers on a high level. There is light at the
end of the tunnel - hope that it isn't the coming train :-)
<Para>
I guess if triggers could also trigger simple select statements, I could
do
most of what I want using triggers except of course the select stuff.
But as I said I like the rules system very much, especially after your
recent
fixes Jan :-) So please stick to supporting all 3: triggers, views and
rules. Wow :-)
<Para>
Well - a trigger cannot build a view. The relation underlying
the view doesn't contain any tuples and a select trigger will
never be fired. As long as there is no possibility to return
tuple sets from non-SQL functions. But a trigger can do
things like the pg_hide_passwd stuff much more powerful. You
could define the trigger so that it checks if the user is a
superuser and overwrite the passwd value only in the case
where he/she isn't. If fired at the right place it would too
work for things like the copy command etc.
<Para>
We must stay with all 3 features. And I will take a look at
the INSERT ... SELECT view problem really soon as it is a
rule system problem that breaks views. But this is only the
SELECT rewriting part of the rule system which I really like
(optimizable). The other areas (insert, update, delete) of
the rule system are dangerous and I really think a powerful
PL/pgSQL language could make them obsolete.
<Sect1>
<Title>Summary from Andreas</Title>
<Para>
Ok, to sum it up:
<ItemizedList Mark="bullet">
<ListItem>
<Para>
We need and want the select part of the rewrite rules.
</Para>
</ListItem>
<ListItem>
<Para>
For the insert/update/delete rules the old instance rules system
was much more appropriate. TODO: dig up the old code
and merge it with the current trigger Implementation;
it must be pretty much the wanted functionality (it
supported SQL).
<Note>
<Title>Vadim's Note</Title>
<Para>
Old instance rules system was removed by Jolly & Andrew and so
it never supported SQL. I hope that Jan will give us PL/pgSQL soon
and it will be used for triggers, without changing current trigger
implementation...
</Para>
</Note>
</Para>
</ListItem>
<ListItem>
<Para>
The CURRENT keyword in the i/u/d rewrite rules is stupid
and should be disabled
destroyed and burned in hell.
<Note>
<Title>Vadim's Note</Title>
<Para>
Agreed, if standard hasn't it. I know that OLD & NEW are in standard,
for triggers atleast.
</Para>
</Note>
</Para>
</ListItem>
<ListItem>
<Para>
To stick to the mainstream we should enhance the trigger
syntax,
and forget the rule stuff for i/u/d
<ProgramListing>
create trigger passwd_utr
..........
referencing old as o new as n
for each row (statement, statement, statement, procedure,
...... all PL/pgSQL syntax allowed );
</ProgramListing>
with a syntax to modify the new tuple in memory.
<Note>
<Title>Vadim's Note</Title>
<Para>
Yes. Statement level triggers give the same functionality as rewrite
i/u/d rules. We could let them to return something special to skip
user' i/u/d itself, isn't it the same as INSTEAD ?
</Para>
</Note>
</Para>
</ListItem>
</ItemizedList>
</Chapter>