JFinis commented on code in PR #514:
URL: https://github.com/apache/parquet-format/pull/514#discussion_r2272753478
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src/main/thrift/parquet.thrift:
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@@ -1082,23 +1093,105 @@ union ColumnOrder {
* BYTE_ARRAY - unsigned byte-wise comparison
* FIXED_LEN_BYTE_ARRAY - unsigned byte-wise comparison
*
- * (*) Because the sorting order is not specified properly for floating
- * point values (relations vs. total ordering) the following
+ * (*) Because the precise sorting order is ambiguous for floating
+ * point types due to underspecified handling of NaN and -0/+0,
+ * it is recommended that writers use IEEE_754_TOTAL_ORDER
+ * for these types.
+ *
+ * If TYPE_ORDER is used for floating point types, then the following
* compatibility rules should be applied when reading statistics:
* - If the min is a NaN, it should be ignored.
* - If the max is a NaN, it should be ignored.
+ * - If the nan_count field is set, a reader can compute
+ * nan_count + null_count == num_values to deduce whether all non-null
+ * values are NaN.
+ * - When looking for NaN values, min and max should be ignored.
+ * If the nan_count field is set, it can be used to check whether
+ * NaNs are present.
* - If the min is +0, the row group may contain -0 values as well.
* - If the max is -0, the row group may contain +0 values as well.
* - When looking for NaN values, min and max should be ignored.
*
* When writing statistics the following rules should be followed:
- * - NaNs should not be written to min or max statistics fields.
+ * - It is suggested to always set the nan_count field for floating
+ * point types, especially also if it is zero.
+ * - NaNs should not be written to min or max statistics fields except
+ * in the column index, where min_values and max_values are not
optional
+ * so a NaN value must be written if all non-null values in a page
+ * are NaN.
* - If the computed max value is zero (whether negative or positive),
* `+0.0` should be written into the max statistics field.
* - If the computed min value is zero (whether negative or positive),
* `-0.0` should be written into the min statistics field.
*/
1: TypeDefinedOrder TYPE_ORDER;
+
+ /*
+ * The floating point type is ordered according to the totalOrder predicate,
+ * as defined in section 5.10 of IEEE-754 (2008 revision). Only columns of
+ * physical type FLOAT or DOUBLE, or logical type FLOAT16 may use this
ordering.
+ *
+ * Intuitively, this orders floats mathematically, but defines -0 to be less
+ * than +0, -NaN to be less than anything else, and +NaN to be greater than
+ * anything else. It also defines an order between different bit
representations
+ * of the same value.
+ *
+ * The formal definition is as follows:
+ * a) If x<y, totalOrder(x, y) is true.
+ * b) If x>y, totalOrder(x, y) is false.
+ * c) If x=y:
+ * 1) totalOrder(−0, +0) is true.
+ * 2) totalOrder(+0, −0) is false.
+ * 3) If x and y represent the same floating-point datum:
+ * i) If x and y have negative sign, totalOrder(x, y) is true if and
+ * only if the exponent of x ≥ the exponent of y
+ * ii) otherwise totalOrder(x, y) is true if and only if the exponent
+ * of x ≤ the exponent of y.
+ * d) If x and y are unordered numerically because x or y is NaN:
+ * 1) totalOrder(−NaN, y) is true where −NaN represents a NaN with
+ * negative sign bit and y is a non-NaN floating-point number.
+ * 2) totalOrder(x, +NaN) is true where +NaN represents a NaN with
+ * positive sign bit and x is a non-NaN floating-point number.
+ * 3) If x and y are both NaNs, then totalOrder reflects a total ordering
+ * based on:
+ * i) negative sign orders below positive sign
+ * ii) signaling orders below quiet for +NaN, reverse for −NaN
+ * iii) lesser payload, when regarded as an integer, orders below
+ * greater payload for +NaN, reverse for −NaN.
+ *
+ * Note that this ordering can be implemented efficiently in software by
bit-wise
+ * operations on the integer representation of the floating point values.
+ * E.g., this is a possible implementation for DOUBLE in Rust:
+ *
+ * pub fn totalOrder(x: f64, y: f64) -> bool {
+ * let mut x_int = x.to_bits() as i64;
+ * let mut y_int = y.to_bits() as i64;
+ * x_int ^= (((x_int >> 63) as u64) >> 1) as i64;
+ * y_int ^= (((y_int >> 63) as u64) >> 1) as i64;
+ * return x_int <= y_int;
+ * }
+ *
+ * When writing statistics for columns with this order, the following rules
+ * must be followed:
+ * - Writing the nan_count field is mandatory when using this ordering,
+ * especialy also if it is zero.
+ * - NaNs should not be written to min or max statistics fields except
Review Comment:
Sure, we can also keep it the same for the column index and statistics here.
My initial idea was to keep the behavior close to what we currently have where
possible. But I agree that we can alos instead make it different from the
current behavior and therefore more consistent between column index and
statistics.
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