[jira] [Created] (AVRO-2620) Max precision on decimal/fixed LogicalType is wrong

["Ryan Skraba (Jira)" <ji...@apache.org>]

Ryan Skraba created AVRO-2620:
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             Summary: Max precision on decimal/fixed LogicalType is wrong
                 Key: AVRO-2620
                 URL: https://issues.apache.org/jira/browse/AVRO-2620
             Project: Apache Avro
          Issue Type: Bug
          Components: java
            Reporter: Ryan Skraba


A decimal logical type with an underlying fixed bytes type is invalid if the precision is greater than can be contained in the fixed bytes.  The calculation for 
 [max precision|https://github.com/apache/avro/blob/a2098bd88361b5ff9298c462a2dccafbb0c2c508/lang/java/avro/src/main/java/org/apache/avro/LogicalTypes.java#L262] can return a wrong value when the fixed bytes size is greater than 128.

For a fixed size of *129*, the `Math.pow(2, 8 * size - 1)` part of this formula will return `Double.INFINITY`, and the max precision will be considered `Long.MAX_VALUE`.  The expected value is 310 digits precision.

We can apply the log power rule to avoid overflow, or some bit-twiddling approximation (like [java.math.BigDecimal|https://github.com/openjdk/jdk/blob/jdk8-b120/jdk/src/share/classes/java/math/BigDecimal.java#L3779].



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