Re: WIP patch (v2) for updatable security barrier views - Mailing list pgsql-hackers

> Kouhei Kaigai <kaigai@ak.jp.nec.com> writes:
> > Let me ask an elemental question. What is the reason why inheritance
> > expansion logic should be located on the planner stage, not on the
> > tail of rewriter?
>
> I think it's mostly historical.  You would however have to think of a way
> to preserve the inheritance relationships in the parsetree representation.
> In the current code, expand_inherited_tables() adds AppendRelInfo nodes
> to the planner's data structures as it does the expansion; but I don't think
> AppendRelInfo is a suitable structure for the rewriter to work with, and
> in any case there's no place to put it in the Query representation.
>
> Actually though, isn't this issue mostly about inheritance of a query
> *target* table?  Moving that expansion to the rewriter is a totally
> different and perhaps more tractable change.  It's certainly horribly ugly
> as it is; hard to see how doing it at the rewriter could be worse.
>
It's just an idea, so might not be a deep consideration.

Isn't ii available to describe a parse tree as if some UPDATE/DELETE statements
are combined with UNION ALL? Of course, even if it is only internal form.
 UPDATE parent SET x = 2*x, y = y || '_update' WHERE x % 10 = 5   UNION ALL UPDATE child_1 SET x = 2*x, y = y ||
'_update'WHERE x % 10 = 5    : 

Right now, only SELECT statement is allowed being placed under set-operations.
If rewriter can expand inherited relations into multiple individual selects
with UNION ALL, it may be a reasonable internal representation.

In similar way,  both of UPDATE/DELETE takes a scan on relation once, then
it modifies the target relation. Probably, here is no significant different
on the earlier half; that performs as if multiple SELECTs with UNION ALL are
running, except for it fetches ctid system column as junk attribute and
acquires row-level locks.

On the other hand, it may need to adjust planner code to construct
ModifyTable node running on multiple relations. Existing code pulls
resultRelations from AppendRelInfo, doesn't it? It needs to take the list
of result relations using recursion of set operations, if not flatten.
Once we can construct ModifyTable with multiple relations on behalf of
multiple relation's scan, here is no difference from what we're doing now.

How about your opinion?

Thanks,
--
NEC OSS Promotion Center / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>


> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: Wednesday, January 29, 2014 3:43 PM
> To: Kaigai, Kouhei(海外, 浩平)
> Cc: Craig Ringer; Simon Riggs; Dean Rasheed; PostgreSQL Hackers; Kohei
> KaiGai; Robert Haas
> Subject: Re: [HACKERS] WIP patch (v2) for updatable security barrier views
>
> Kouhei Kaigai <kaigai@ak.jp.nec.com> writes:
> > Let me ask an elemental question. What is the reason why inheritance
> > expansion logic should be located on the planner stage, not on the
> > tail of rewriter?
>
> I think it's mostly historical.  You would however have to think of a way
> to preserve the inheritance relationships in the parsetree representation.
> In the current code, expand_inherited_tables() adds AppendRelInfo nodes
> to the planner's data structures as it does the expansion; but I don't think
> AppendRelInfo is a suitable structure for the rewriter to work with, and
> in any case there's no place to put it in the Query representation.
>
> Actually though, isn't this issue mostly about inheritance of a query
> *target* table?  Moving that expansion to the rewriter is a totally
> different and perhaps more tractable change.  It's certainly horribly ugly
> as it is; hard to see how doing it at the rewriter could be worse.
>
>             regards, tom lane