Explain why floats are different with respect to indexing

minisql/src/type_system.rs

@@ -15,8 +15,20 @@ pub type Uuid = u64;
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub enum Value {
-    Number(f64), // TODO: Can't put floats as keys in maps, since they don't implement Eq. What to
+    // Note that it doesn't really make sense to compare floats on equality without specifying
-    // do?
+    // precision. You can ofcourse convert a float to string or to a bytevector and then compare
+    // equality of those, but that's not the right equality. And ofcourse Rust designers are aware
+    // of this, so floats don't implement the Eq trait.
+    // This ofcourse complicates indexing of Number columns.
+    //
+    // Either we'd have to design a specific key-value map data-structure where keys are floats,
+    // s.t. to index with a given float K you also specify a tolerance error so that the resulting
+    // value set will contain all values whose keys are close to K within that tolerence. This
+    // seems highly non-trivial.
+    //
+    // So we choose to make a distinction between indexable and non-indexable types, and Number is
+    // not indexable.
+    Number(f64),
     Indexable(IndexableValue),
 }