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Re: Bad query plan when the wrong data type is used

From: Vitalii Tymchyshyn <tivv00(at)gmail(dot)com>
To: Dave Crooke <dcrooke(at)gmail(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance(at)postgresql(dot)org, Daniel Fekete <danieleff(at)gmail(dot)com>
Subject: Re: Bad query plan when the wrong data type is used
Date: 2011-02-09 09:52:44
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
09.02.11 01:14, Dave Crooke написав(ла):
> You will get the same behaviour from any database product where the 
> query as written requires type coercion - the coercion has to go in 
> the direction of the "wider" type. I have seen the exact same scenario 
> with Oracle, and I view it as a problem with the way the query is 
> written, not with the database server.
> Whoever coded the application which is making this query presumably 
> knows that the <> field is an integer type in 
> the schema they designed, so why are they passing a float? Convert the 
> 4.0 to 4 on the application side instead, it's one function call or cast.
Actually the problem may be in layers, and the  problem may even be not 
noticed until it's late enough. As far as I remember from this list 
there are problems with column being integer and parameter prepared as 
bigint or number. Same for number vs double vs float.
As for me it would be great for optimizer to consider the next:
1) val1::narrow = val2::wide as (val1::narrow = val2::narrow and 
val2::narrow = val2::wide)
2) val1::narrow < val2::wide as (val1::narrow < val2::narrow and 
val1::wide < val2::wide)
3) val1::narrow > val2::wide as (val1::narrow + 1 > val2::narrow and 
val1::wide > val2::wide)
Of course it should use additional check it this allows to use an index.
Surely, this is not an easy thing to implement, but as for me similar 
question are raised quite often in this list.

Best regards, Vitalii Tymchyshyn

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