feat: Allow log with non-integer base on decimals

Author: Yuvraj-cyborg

datafusion/functions/src/math/log.rs

@@ -102,32 +102,39 @@ impl LogFunc {
     }
 }
 
-/// Binary function to calculate an integer logarithm of Decimal128 `value` using `base` base
-/// Returns error if base is invalid
 fn log_decimal128(value: i128, scale: i8, base: f64) -> Result<f64, ArrowError> {
-    if !base.is_finite() || base.trunc() != base {
+    if !base.is_finite() {
         return Err(ArrowError::ComputeError(format!(
-            "Log cannot use non-integer base: {base}"
+            "Log cannot use non-finite base: {base}"
         )));
     }
-    if (base as u32) < 2 {
+    if base <= 1.0 {
         return Err(ArrowError::ComputeError(format!(
             "Log base must be greater than 1: {base}"
         )));
     }
 
-    let unscaled_value = decimal128_to_i128(value, scale)?;
-    if unscaled_value > 0 {
-        let log_value: u32 = unscaled_value.ilog(base as i128);
-        Ok(log_value as f64)
+    // For integer bases >= 2, use the efficient integer algorithm
+    if base.trunc() == base && base >= 2.0 && base <= u32::MAX as f64 {
+        let unscaled_value = decimal128_to_i128(value, scale)?;
+        if unscaled_value > 0 {
+            let log_value: u32 = unscaled_value.ilog(base as i128);
+            return Ok(log_value as f64);
+        } else {
+            return Ok(f64::NAN);
+        }
+    }
+
+    // For non-integer bases, fallback to f64 computation
+    let scale_factor = 10f64.powi(scale as i32);
+    let value_f64 = (value as f64) / scale_factor;
+    if value_f64 > 0.0 {
+        Ok(value_f64.log(base))
     } else {
-        // Reflect f64::log behaviour
         Ok(f64::NAN)
     }
 }
 
-/// Binary function to calculate an integer logarithm of Decimal128 `value` using `base` base
-/// Returns error if base is invalid or if value is out of bounds of Decimal128
 fn log_decimal256(value: i256, scale: i8, base: f64) -> Result<f64, ArrowError> {
     match value.to_i128() {
         Some(value) => log_decimal128(value, scale, base),

datafusion/sqllogictest/test_files/decimal.slt

@@ -878,14 +878,20 @@ select log(2, 100000000000000000000000000000000000::decimal(38,0));
 ----
 116
 
-# log(10^35) for decimal128 with another base
+# log(10^35) for decimal128 with another base (float base)
 # TODO: this should be 116.267483321058, error with native decimal log impl
 #       https://github.com/apache/datafusion/issues/18524
 query R
 select log(2.0, 100000000000000000000000000000000000::decimal(38,0));
 ----
 116
 
+# log with non-integer base now works (fallback to f64)
+query R
+select log(2.5, 100::decimal(38,0));
+----
+5.025883189464
+
 # null cases
 query R
 select log(null, 100);