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Re: array support patch phase 1 patch

From: Joe Conway <mail(at)joeconway(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Patches (PostgreSQL)" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: array support patch phase 1 patch
Date: 2003-03-25 23:55:00
Message-ID: 3E80EC54.2040203@joeconway.com (view raw or flat)
Thread:
Lists: pgsql-patches
Tom Lane wrote:
> I looked over this patch a little bit, mostly at the anyarray/anyelement
> changes; I didn't study the ArrayExpr stuff (except to notice that yeah,
> the grammar change is very ugly; can't it be made independent of the
> number of levels?).

I struggled with it for quite a while, but then again, I can't claim 
yacc grammar as a particularly strong point. I kept running into 
reduce/reduce errors with anything completely recursive, or when it did 
work, I found myself restricted to one level of nesting. I'll try again 
-- any suggestions for something similar to study?

> parameter types could be made visible to the function.  I see your concern
> here, but I think the only adequate solution is to make sure the function
> can learn the types of all its arguments, as well as the assigned return
> type.  (Of course it could deduce the latter from the former, but we may
> as well save it the effort, especially since it would have to repeat
> parse-time catalog lookups in many interesting cases.)

I came to the same conclusion with ArrayExpr/ArrayExprState -- you can 
always deduce the array type from the element type and vice-versa, but 
it seemed costly to do in the executor when it can be done just once in 
the parser.

> A notion that I've toyed with more than once is to allow a function
> to get at the parse tree representation for its call (ie, the FuncExpr
> or OpExpr node).  If we did that, a function could learn all these
> types by examining the parse tree, and it could learn other things
> besides.  A disadvantage is that functions would have to be ready to
> cope with all the wooliness of parse trees.  It'd probably be bright
> to make some convenience functions "get my return type", "get type
> of my n'th argument" to localize this logic as much as possible.

OK -- I'll take a look at that option.


> In short then, what about passing an Expr* link in FmgrInfo as a
> substitute for actualRetType?  (An additional advantage of this way
> is that it's obvious whether or not the information has been provided,
> which it would not be for, say, functions invoked via DirectFunctionCallN.
> The patch as it stands fails silently if a polymorphic function is called
> from a call site that doesn't provide the needed info.)

Sounds good to me -- thanks for the quick feedback!

A question -- I was thinking that we will need to allow one array type 
to be coerced into another in this brave new world (e.g. 
ARRAY[1,2,3]::float8[] results in "ERROR:  Cannot cast type integer[] to 
double precision[]"). My plan was to check the source and target 
datatypes in can_coerce_type() and coerce_type() to see if both are 
array types. If so, use the element types to determine whether we can 
coerce, and loop over the array to coerce, etc.

There are a few issues that I can think of with this (and undoubtedly 
you will have some others ;-)):
  - this would add at least one cache lookup to every call to
    can_coerce_type
  - if we pass by can_coerce_type with an array, we'd need to do the same
    cache lookup in coerce_type unless we modify the api of the related
    functions to pass along the "isarray" status of the datatype
  - currently the only reliable(?) method to determine if a
    datatype is a varlena array is to check for '_' as the first
    character of the type name -- it seems wrong to depend on that
    forever.

Any suggestions? I was toying with the idea that an isarray attribute 
should be added to pg_type -- thoughts on that?

Joe


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