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Re: explain and PARAM_EXEC

From: Yeb Havinga <yebhavinga(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: explain and PARAM_EXEC
Date: 2010-04-02 15:21:37
Message-ID: (view raw or flat)
Lists: pgsql-hackers
Tom Lane wrote:
> Anyway, I hadn't looked at your patch before, but now that I have, it's
> not even approximately what I was suggesting.  What I thought you should
> do was change ruleutils.c to print the parameter expressions at the call
> site, ie in the T_SubPlan and T_AlternativeSubPlan cases in get_rule_expr.
See patch below against HEAD.

Example of query against catalog:

postgres=# explain verbose select oid::int + 1,(select oid from pg_class 
a where a.oid = b.relfilenode and a.relnamespace = b.relnamespace) from 
pg_class b;
 Seq Scan on pg_catalog.pg_class b  (cost=0.00..2459.64 rows=296 width=12)
   Output: ((b.oid)::integer + 1), SubPlan 1 ($0 := b.relfilenode, $1 := 
   SubPlan 1
     ->  Index Scan using pg_class_oid_index on pg_catalog.pg_class a  
(cost=0.00..8.27 rows=1 width=4)
           Output: a.oid
           Index Cond: (a.oid = $0)
           Filter: (a.relnamespace = $1)
(7 rows)

Explain of alternative subplan (query comes from aggregates.sql in 

regression=# explain verbose select ten, sum(distinct four) from onek a
group by ten
having exists (select 1 from onek b where sum(distinct a.four) = b.four);
 GroupAggregate  (cost=197.83..216.52 rows=10 width=8)
   Output: a.ten, sum(DISTINCT a.four)
   Filter: (alternatives: SubPlan 1 ($0 := sum(DISTINCT a.four)) or 
hashed SubPlan 2)
   ->  Sort  (cost=197.83..200.33 rows=1000 width=8)
         Output: a.ten, a.four
         Sort Key: a.ten
         ->  Seq Scan on public.onek a  (cost=0.00..148.00 rows=1000 
               Output: a.ten, a.four
   SubPlan 1
     ->  Seq Scan on public.onek b  (cost=0.00..150.50 rows=250 width=0)
           Filter: ($0 = b.four)
   SubPlan 2
     ->  Seq Scan on public.onek b  (cost=0.00..148.00 rows=1000 width=4)
           Output: b.four
(14 rows)

Would the explain above be better if the filter with subplans arguments 
showed the EXISTS keyword? Much code from get_sublink_expr could be 
reused to show additional info depending on the sublink type and testexpr.

Other info: there are now only appends to the context->buf. The problems 
with OUTER var I mentioned earlier are gone now arguments are deparsed 
using get_rule_expr instead of deparse_expression.

Yeb Havinga

Attachment: subplanarguments.patch
Description: text/plain (3.0 KB)

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