shifluxxc opened a new pull request, #19315:
URL: https://github.com/apache/datafusion/pull/19315
## Which issue does this PR close?
- Closes #19250
## Rationale for this change
Previously, the `log` function would fail when operating on decimal values
with negative scales.
Negative scales in decimals represent values where the scale indicates
padding zeros to the right (e.g., `Decimal128(38, -2)` with value `100`
represents `10000`). This PR restores support for negative-scale decimals in
the `log` function by implementing the logarithmic property: `log_base(value *
10^(-scale)) = log_base(value) + (-scale) * log_base(10)`.
## What changes are included in this PR?
1. **Enhanced `log_decimal128` function**:
- Added support for negative scales using the logarithmic property
- For negative scales, computes `log_base(value) + (-scale) *
log_base(10)` instead of trying to convert to unscaled value
- Added detection for negative-scale decimals in both the number and base
arguments
- Skips simplification when negative scales are detected to avoid errors
with `ScalarValue` (which doesn't support negative scales yet)
2. **Added comprehensive tests**:
- Unit tests in `log.rs` for negative-scale decimals with various bases
(2, 3, 10)
- SQL logic tests in `decimal.slt` using scientific notation (e.g.,
`1e4`, `8e1`) to create decimals with negative scales
## Are these changes tested?
Yes, this PR includes comprehensive tests:
1. Unit tests:
- `test_log_decimal128_negative_scale`: Tests array inputs with negative
scales
- `test_log_decimal128_negative_scale_base2`: Tests with base 2 and
negative scales
- `test_log_decimal128_negative_scale_scalar`: Tests scalar inputs with
negative scales
2. SQL logic tests:
- Tests for unary log with negative scales (`log(1e4)`)
- Tests for binary log with explicit base 10 (`log(10, 1e4)`)
- Tests for binary log with base 2 (`log(2.0, 8e1)`, `log(2.0, 16e1)`)
- Tests for different negative scale values (`log(5e3)`)
- Tests for array operations with negative scales
- Tests for different bases (2, 3, 10) with negative-scale decimals
All tests pass successfully.
## Are there any user-facing changes?
Yes, this is a user-facing change:
**Before**: The `log` function would fail with an error when operating on
decimal values with negative scales:
```sql
-- This would fail
SELECT log(1e4); -- Error: Negative scale is not supported
```
**After**: The `log` function now correctly handles decimal values with
negative scales:
```sql
-- This now works
SELECT log(1e4); -- Returns 4.0 (log10(10000))
SELECT log(2.0, 8e1); -- Returns ~6.32 (log2(80))
```
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