Hello Pavel,
As a follow-up to the \if patch by Corey Huinker, here is a proposal to
allow testing whether a client-side variable exists in psql.
The syntax is as ugly as the current :'var' and :"var" things, but ISTM
that this is the only simple option to have a working SQL-compatible syntax
with both client-side substitution and server side execution. See the
second example below.
It is really ugly
Yes, I agree:-)
- the ? symbol is not used in pair usually - so it is
much more visible - it is bad readable.
Yep.
Maybe some other syntax: :{fx xxx} .. where fx can be one from more
possible operators ? ! ...
Any colon prefixed syntax can be made to work because it is enough for
the lexer to detect and handle... so
:{defined varname}
may be an option, although I do not like the space much because it adds
some fuzzyness in the lexer which has to process it. It is probably
doable, though. I like having a "?" because there is a question. Other
suggestions somehow in line with your proposal could be
:{?varname}
:{varname?}
what do you think?
The other option would be to have some special keyword syntax, say
"defined var", but then it would have to be parsed client side, and how to
do that in an SQL expression is unclear, and moreover it would not look
right in an SQL expression. If it would look like a function call, say
"defined('var')", it would potentially interact with existing server-side
user-defined functions, which is pretty undesirable. Hence the :?...?
proposal above which is limited to variable subsitution syntax.
should not be solved by introduction \ifdef ?
This would be a client side only non extendable option: you can only test
one variable at a time, you cannot say "NOT" or have to do \ifndef... CPP
started like that and ended with #if bool-expr-with-defined in the end.
The idea is to extend the newly added \if with client-side SQL-compatible
expression syntax, and that the same syntax could be used server side with
select, eg:
-- client side
\let i :j + 1
-- server side
SELECT :j + 1 AS i \gset
-- client-side conditional
\if NOT :j > 1 OR ...
The colon prefixed substitution syntax already works for server side,
but there is no way to check whether a variable exists which is
compatible with that, hence this patch.
Pgbench has expressions with patches to improve it, especially adding
boolean operators. I think that the simplest plan is, when stabalized, to
move the parser & executir to fe_utils and to use it from both psql et
pgbench. Also I plan to add \if to pgbench, possibly by factoring some of
the code from psql in fe_utils as well because it would help with
benchmarks.
Now given the patch queue length and speed I'm not even thinking of
starting doing all this.
--
Fabien.
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