Re: proposal: doc: simplify examples of dynamic SQL

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От David G. Johnston
Тема Re: proposal: doc: simplify examples of dynamic SQL
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Msg-id CAKFQuwY+40SPc33F-BkL3-8LAcwMvbn0kpxZOCF2MU8XpXOJwQ@mail.gmail.com
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Ответ на Re: proposal: doc: simplify examples of dynamic SQL  (Bruce Momjian <bruce@momjian.us>)
Ответы Re: proposal: doc: simplify examples of dynamic SQL  (Bruce Momjian <bruce@momjian.us>)
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On Thu, Mar 19, 2015 at 5:18 PM, Bruce Momjian <bruce@momjian.us> wrote:
On Thu, Mar 19, 2015 at 04:01:32PM -0700, David G. Johnston wrote:
> ​Prefacing it with:  "You may also see the following syntax in the wild since
> format was only recently introduced."​
>  
> ​may solve your lack of reason for inclusion.

Uh, the problem with that is we are not going to revisit this when
format isn't "recently introduced".  I think script writers naturally
think of query construction using string concatenation first, so showing
it first seems fine.


​+1​

There are other places later in the docs where we explain all the quote*
functions and show examples of query construction using string
concatenation, but I am not sure how we can remove those.


​Can you be more specific?

On a related note:

"If you are dealing with values that might be null, you should usually use quote_nullable in place of quote_literal."

Its unclear why, aside from semantic uncleanliness, someone would use quote_literal given its identical behavior for non-null values and inferior behavior which passed NULL.  The function table for the two could maybe be more clear since quote_nullable(NULL) returns a string representation of NULL without any quotes while quote_literal(NULL) returns an actual NULL that ultimately poisons the string concatenation that these functions are used with.

<reads some more>

The differences between the actual null and the string NULL are strictly in capitalization - which is not consistent even within the table.  concat_ws states "NULL arguments are ignored" and so represents actual null with all-caps which is string NULL in the quote_* descriptions.  Having read 40.5.4 and example 40-1 the difference is clear and obvious so maybe what is in the table is sufficient for this topic.

I would suggest adding a comment to quote_ident and quote_nullable that corresponding format codes are %I and %L.  Obviously there is no "quote_" function to correspond with %S.  There is likewise nor corresponding format code for quote_literal since quote_nullable is superior in every way (that I can tell at least).

David J.

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