Merge pull request #19 from UnityHPC/fix/db_connection_custom_query

Custom query and preprocess missing columns handling

Author: bpachev

docs/preprocess.md

@@ -4,14 +4,16 @@
 ### Attributes Omitted
 - **UUID**
 - **Nodes**: NodesList have more specific information
-- **Preempted**: Status have more valid information
+- **Preempted**: Contains unreliable data. Use Status column instead (PREEMPT for
+    unfinished, COMPLETE/FAILED/etc. for finished preempted jobs).
 - **EndTime**: Can be calculated from StartTime and Elapsed
 
 ### Options for Including or Omitting Jobs
 - **Keeping CPU jobs:**
     - If `GPUType` is null, the value will be filled with `["cpu"]`
     - If `GPUs` is null or is 0, the value will be 0.
 - **Keeping jobs where the status is "Failed" or "Cancelled"**
+- **Keeping jobs where the QOS is customized (not normal, long, or short)**
 
 ### Records Omitted If:
 - `Elapsed` is less than the minimum threshold