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Convert boolean case expressions to boolean logic #1719

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Feb 1, 2022
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Convert boolean case expressions to boolean logic #1719

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Convert boolean case expressions to boolean logic
tustvold Jan 31, 2022
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tustvold Feb 1, 2022
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123 changes: 120 additions & 3 deletions datafusion/src/optimizer/simplify_expressions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -662,6 +662,54 @@ impl<'a> ExprRewriter for Simplifier<'a> {
_ => unreachable!(),
},

//
// Rules for Case
//

// CASE
// WHEN X THEN A
// WHEN Y THEN B
// ...
// ELSE Q
// END
//
// ---> (X AND A) OR (Y AND B AND NOT X) OR ... (NOT (X OR Y) AND Q)
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//
// Note: the rationale for this rewrite is that the expr can then be further
// simplified using the existing rules for AND/OR
Case {
expr: None,
when_then_expr,
else_expr,
} if !when_then_expr.is_empty()
&& when_then_expr.len() < 3 // The rewrite is O(n!) so limit to small number
&& self.is_boolean_type(&when_then_expr[0].1) =>
{
// The disjunction of all the when predicates encountered so far
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@alamb alamb Jan 31, 2022 •
edited
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Given the complexity of expression this could potentially produce O(n!) maybe in terms of the number of when exprs (I think?), what do you think about adding a heuristic that only does this rewrite when there are a 'small' number of cases -- perhaps 2 or 3?

This would prevent some sort of pathological explosion and would handle the usecase of simple mapping CASE statements

let mut filter_expr = lit(false);
// The disjunction of all the cases
let mut out_expr = lit(false);

for (when, then) in when_then_expr {
let case_expr = when
.as_ref()
.clone()
.and(filter_expr.clone().not())
.and(*then);

out_expr = out_expr.or(case_expr);
filter_expr = filter_expr.or(*when);
}

if let Some(else_expr) = else_expr {
let case_expr = filter_expr.not().and(*else_expr);
out_expr = out_expr.or(case_expr);
}

// Do a first pass at simplification
out_expr.rewrite(self)?
}

expr => {
// no additional rewrites possible
expr
Expand Down Expand Up @@ -1175,6 +1223,8 @@ mod tests {
.expect("expected to simplify")
.rewrite(&mut const_evaluator)
.expect("expected to const evaluate")
.rewrite(&mut rewriter)
.expect("expected to simplify")
}

fn expr_test_schema() -> DFSchemaRef {
Expand Down Expand Up @@ -1291,6 +1341,11 @@ mod tests {

#[test]
fn simplify_expr_case_when_then_else() {
// CASE WHERE c2 != false THEN "ok" == "not_ok" ELSE c2 == true
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Unfortunately the optimizer is not especially brilliant at optimizing boolean expressions - I've created #1716 to track this.

I'm not sure if we want to constrain the rewrite until the boolean expression optimizer is able to more effectively reduce down what it produces...

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🤔 I do wonder given the nullability concerns, if in addition to "simplify" which seeks to retain the same semantics, there could be something like:

/// returns true if this expr *always* returns null or false
/// (aka would filter out all rows from a query)
fn is_null_or_false(expr: &Expr) -> bool {
  ...
}

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I'm not sure I follow, are you suggesting I change something or just musing 😅

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Sorry -- I was musing -- no specific changes suggested

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I implemented something related here: https://github.com/influxdata/influxdb_iox/pull/3557/files#r796984073

// -->
// CASE WHERE c2 THEN false ELSE c2
// -->
// false
assert_eq!(
simplify(Expr::Case {
expr: None,
Expand All @@ -1300,11 +1355,73 @@ mod tests {
)],
else_expr: Some(Box::new(col("c2").eq(lit(true)))),
}),
Expr::Case {
col("c2").not().and(col("c2")) // #1716
);

// CASE WHERE c2 != false THEN "ok" == "ok" ELSE c2
// -->
// CASE WHERE c2 THEN true ELSE c2
// -->
// c2
assert_eq!(
simplify(Expr::Case {
expr: None,
when_then_expr: vec![(Box::new(col("c2")), Box::new(lit(false)))],
when_then_expr: vec![(
Box::new(col("c2").not_eq(lit(false))),
Box::new(lit("ok").eq(lit("ok"))),
)],
else_expr: Some(Box::new(col("c2").eq(lit(true)))),
}),
col("c2").or(col("c2").not().and(col("c2"))) // #1716
);

// CASE WHERE ISNULL(c2) THEN true ELSE c2
// -->
// ISNULL(c2) OR c2
assert_eq!(
simplify(Expr::Case {
expr: None,
when_then_expr: vec![(
Box::new(col("c2").is_null()),
Box::new(lit(true)),
)],
else_expr: Some(Box::new(col("c2"))),
}
}),
col("c2")
.is_null()
.or(col("c2").is_null().not().and(col("c2")))
);

// CASE WHERE c1 then true WHERE c2 then false ELSE true
// --> c1 OR (NOT(c1) AND c2 AND FALSE) OR (NOT(c1 OR c2) AND TRUE)
// --> c1 OR (NOT(c1 OR c2))
// --> NOT(c1) AND c2
assert_eq!(
simplify(Expr::Case {
expr: None,
when_then_expr: vec![
(Box::new(col("c1")), Box::new(lit(true)),),
(Box::new(col("c2")), Box::new(lit(false)),)
],
else_expr: Some(Box::new(lit(true))),
}),
col("c1").or(col("c1").or(col("c2")).not())
);

// CASE WHERE c1 then true WHERE c2 then true ELSE false
// --> c1 OR (NOT(c1) AND c2 AND TRUE) OR (NOT(c1 OR c2) AND FALSE)
// --> c1 OR (NOT(c1) AND c2)
// --> c1 OR c2
assert_eq!(
simplify(Expr::Case {
expr: None,
when_then_expr: vec![
(Box::new(col("c1")), Box::new(lit(true)),),
(Box::new(col("c2")), Box::new(lit(false)),)
],
else_expr: Some(Box::new(lit(true))),
}),
col("c1").or(col("c1").or(col("c2")).not())
);
}

Expand Down