[CASSANDRA-15796] Cassandra get range slice got Inaccurate estimates for concurrencyFactor

• Type: Bug
• Status: Open
• Priority: Normal
• Resolution: Unresolved
• Component/s: Legacy/Coordination
• Bug Category: Correctness
• Severity: Normal
• Complexity: Normal
• Discovered By: User Report
• Platform: All
• Assignee: Unassigned
• Reporter: Maxwell Guo
• Created: 07/May/20 10:09
• Updated: 21/Mar/21 23:36

Description

Cassandra version :3.11.4
For table that do not got a regular column , when we do partition range read and we got to the code function :

// Some comments here
public String getRangeSlice(PartitionRangeReadCommand command, ConsistencyLevel consistencyLevel, long queryStartNanoTime)

we will do some calculation to estimate the data per range in the function :

// Some comments here
private static float estimateResultsPerRange(PartitionRangeReadCommand command, Keyspace keyspace)

when we got no index so we will use :

// Some comments here
command.limits().estimateTotalResults(cfs)
 --->(for CQLlimits)
*public float estimateTotalResults(ColumnFamilyStore cfs)
        {
            // TODO: we should start storing stats on the number of rows (instead of the number of cells, which
            // is what getMeanColumns returns)
            float rowsPerPartition = ((float) cfs.getMeanColumns()) / cfs.metadata.partitionColumns().regulars.size();
            return rowsPerPartition * (cfs.estimateKeys());
        }*

when the regulars is zero ,we will got an error result that is non-zero value divided by zero .