Re: [HACKERS] Variable substitution in psql backtick expansion
| От | Pavel Stehule |
|---|---|
| Тема | Re: [HACKERS] Variable substitution in psql backtick expansion |
| Дата | |
| Msg-id | CAFj8pRC=inphd=S16KjGqq+RcmR+YBMAts0JxTDg8VGp8itRzg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: [HACKERS] Variable substitution in psql backtick expansion (Fabien COELHO <coelho@cri.ensmp.fr>) |
| Список | pgsql-hackers |
2017-04-11 9:07 GMT+02:00 Fabien COELHO <coelho@cri.ensmp.fr>:
I think so implementation of simple expression evaluation should not be
hard
Indeed it is not hard, it is rather a matter of deciding what it should do, and the syntax to do it.- really just - we can expect so any variable will be replaced by
const in expression
Num (<|>|=|<=|>=) Num
<> and != would seem handy as well.
sorry - I forgot
Text (<|>|=|<=|>=) Text
What would be the use case for handling TEXT?not Bool, Bool (or|and) Bool
Aka logical expressions.and special operator "defined"
I'm still not buying this suggestion at all because it does not look like SQL and I think that client-side expressions should be a simple subset of SQL expressions, which a "defined" operators is definitely not.
The "defined" tests is not coming from SQL universe. It is coming from scripting systems - In plain SQL I can use IS NULL. When I check any not existing variable in plpgsql I expect syntax error. So SQL doesn't know any similar to "defined" and it is ok. Currently In psql it is similar. When I use undefined psql variable I got syntax error. When I expect so some content of command line will come from command line or (possible) from some interactive action I would to handle this situation to by my script more user friendly - and I can write more user friendly error messages or I can react on it - enforce user input.
I cannot do test on client side test on NULL - currently psql variables doesn't support it - and I am think so it is not what I want - I am interesting about some meta information from outside.
else
I need to check if I can use some psql variable. I have to do on client side. In some languages is usual term defined - some other using some special syntax or special environments.
The my proposal "defined variablename" should be simple on implementation, but should not be one. It is just proposal.
The tests:
variable is defined, variable is null, ... is acceptable for me too - although I have small problem with NULL, because NULL can got from server - more psql variables doesn't support NULL, and support can enforce incompatible change.
Hmmm. I'm not sure I get it. The penalty I see is that it adds a dummy
variable which must be given a sensible name, and for very short
expressions this is not a win. But this is a minor point.I know so it is not ideal - but the language with commands "\if", "\else"
... is not ideal language.
Sure.
--
Fabien.
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