Just to mention, you can also force a tuple-at-the-time evaluation in pure SQL:

-- BEGIN

declare cs boolean;
set cs=true;

START TRANSACTION;

CREATE OR REPLACE FUNCTION mycontains(s STRING, p STRING, casesensitive boolean)
RETURNS BOOLEAN
BEGIN
  IF casesensitive
  THEN RETURN SELECT s like '%'||p||'%';
  ELSE RETURN SELECT s ilike '%'||p||'%';
  END IF;
END;

CREATE TABLE t(s STRING);
INSERT INTO t VALUES ('apple'),('pear'),('banana'),('orange');

explain select * from t where mycontains(s,'an',cs);;

-- END

If you look at this explain, you'll see it makes a loop that calls the mycontains() function. This does avoid evaluating both branches. However the explicit interpreted loop won't make it very fast, perhaps see on your data. (Note: when the function is simpler, the loop is actually done in C)

On Mon, 7 Oct 2019 at 16:53, Daniel Zvinca <daniel.zvinca@logbis.com> wrote:

I wasn't sure if this question should be posted here or on regular user list, considering the way statements are generated and possible connections with internal functionality.

I understand the explanations. I should have checked the explain feature myself. I think that I expected the optimizer to notice a constant expression and evaluate it before choosing which real columnar operation needs to be performed (obviously only one). Instead all members are evaluated, the whole explain statement is self-explanatory, indeed. (a delayed bat evaluation mechanism would have helped, I guess, for this case)

I use capi intensively, but for string columns the capi memory management can be a serious limitation for large sets.

Anyway, your answer is very helpful for me, it makes me consider adjusting my approach from a straight translation into one SQL statement into using an intermediary language that would build the optimal statement instead of stretching the SQL to get what I need.

Thank you,

On Mon, Oct 7, 2019 at 4:44 PM Sjoerd Mullender <sjoerd@monetdb.org> wrote:

On 07/10/2019 14.33, Roberto Cornacchia wrote:
> Hi Daniel,

[...]

> In this case, you would get better performance by implementing a BAT
> function (not a function that works on a single value, but on a column
> of values). 
> You would write that in C using the GDK api
> (https://www.monetdb.org/Documentation/Cookbooks/SQLrecipes/UserDefinedFunction),
> and evaluate the condition i the loop, per value.
> However, this requires you to recompile MonetDB to include your new
> function.

Recompilation of MonetDB should not be necessary.  See [1].

[1] https://www.monetdb.org/hg/MonetDB-extend/

--
Sjoerd Mullender

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