[WIP] Native SQLite Database reader (works at CTFE)

[Stefan Koch via Digitalmars-d]

On Sunday, 21 February 2016 at 18:32:29 UTC, Chris Wright wrote:
> Or rather, parsing a query to use an index becomes exponential 
> unless you parse X86 instructions.
>
> D doesn't let you access the contents of a lambda expression. 
> (C# does.) So in order to evaluate the lambda, you have to 
> invoke it.
>
> You can't override <= separately from <. You only have an opCmp 
> invocation and return value. So with a query filter like:
>
> query.where!("age", (age) => age >= 18);
>
> You know it's being compared to 10, but you don't know what 
> comparison. So the first time, you have opCmp return -1, and 
> the lambda yields false. Try again, have opCmp return 0, and 
> the lambda yields true. Try a third time, have opCmp return 1, 
> and it yields true.
>
> That's three invocations for one variable. What about two 
> variables?
>
> query.where!("age,parental_supervision",
>   (age, parentalSupervision) => age >= 18 || 
> parentalSupervision == true);
>
> Now I have to try three different values for the 'age' 
> expression and two for the 'parentalSupervision' variable, and 
> there's a combinatorial expansion.
>
> That yields a giant truth table, and that's probably good 
> enough. You look at which columns have an index and, of those, 
> which have the best correlation with the lambda's result. That 
> even lets you reduce the amount of memory you use -- you don't 
> need to hold the whole truth table, just a series of counters. 
> But you're still dealing with O(2**n) evaluations of the lambda.

I don't parse anything.
the string "age" gets evaluated once so I know which columns your 
lambda is referring to
then the lambda gets evaluated exactly n times where n is the 
number of rows.
I your example above you should have written :
query.where!("age","parentalSupervision", (age,pa) => age.as!uint 
 >= 18 || pa == true)