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75 lines
3.4 KiB
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From pgsql-general-owner+M10387@postgresql.org Mon Jun 4 22:02:55 2001
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Mon, 4 Jun 2001 21:42:58 -0400 (EDT)
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To: Rasmus Resen Amossen <spunk@rhk.dk>
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cc: pgsql-general@postgresql.org
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Subject: Re: [GENERAL] Re: Updating views
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In-Reply-To: <3B1C16EC.8D9FB57B@rhk.dk>
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References: <20010605001048.A2133@lorien.net> <3B1C16EC.8D9FB57B@rhk.dk>
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Comments: In-reply-to Rasmus Resen Amossen <spunk@rhk.dk>
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message dated "Tue, 05 Jun 2001 01:17:00 +0200"
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Date: Mon, 04 Jun 2001 21:42:57 -0400
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Message-ID: <9925.991705377@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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Rasmus Resen Amossen <spunk@rhk.dk> writes:
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> OK, but I can't see how to make a single rule that allows me to update
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> an arbitray set of attributes from an arbitray where-clause.
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The reason the system doesn't do that for you is that it's *hard* to
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figure out what to do for an arbitrary where-clause. An automatic rule
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has no chance of doing the right thing, because the right thing depends
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on what you intend. For example, if your view has
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select ... where a>5;
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what do you think ought to happen if someone tries to insert a row
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with a<5? Is that an error? A no-op? Does the row go in anyway,
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you just can't see it in the view? Does the row go into some other
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table instead? Is it OK to change the A column at all? It all depends
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on the semantics of your database design. So you have to figure out
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what you want and write rules that do it.
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The mechanics of the rule are not that painful once you've decided what
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the reverse mapping from inserted/updated data to underlying tables
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ought to be. One thing that may help is to realize that you don't need
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a separate rule for each combination of set of attributes that might be
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updated. "new.*" is defined for all columns including the ones that
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didn't change, so you can just do something like
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update ... set f1 = new.f1, f2 = new.f2, ...
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without worrying about just which columns the user tried to update.
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Likewise, the where clause in the user's query is not yours to worry
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about; that condition gets added onto the stuff in your rule.
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> In other words: I want to make the update of 'exview' transparent to
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> 'extable'.
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If it's really transparent, one wonders why you bothered with a view
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at all. Useful views tend to be nontrivial mappings of the underlying
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data, which is why it's nontrivial to figure out what the reverse
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mapping ought to be.
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regards, tom lane
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