Impala query performance issues

I am running a Query which returns 5 rows
select distinct date_key from tbl_date limit 5; /the table has a few hundred rows with 1 partition/

The Query info is

Planning Wait Time: 18.8m Planning Wait Time Percentage: 100

Query Timeline Start execution: 95.55us (95.55us)
Planning finished: 18.8m (18.8m)
Submit for admission: 18.8m (345.26us)
Completed admission: 18.8m (107.67us)
Ready to start remote fragments: 18.8m (1ms)
Remote fragments started: 18.8m (1.13s)
Rows available: 18.9m (4.15s)
First row fetched: 18.9m (7ms)
Unregister query: 18.9m (33ms)

The issue is why is the "Planning Wait Time" so high ?
planning wait time is defined as - The total amount of time the query spent waiting for planning to complete.

what I don't understand with such a simple query why is spending so much time planning ?
also a lot of queries which are slow have high Planning Wait Time Percentage why ?
can I get some more info on Planning Wait Time metric?

Query Info -
Start Time: Nov 30, 2016 11:17:13 AM
End Time: Nov 30, 2016 11:36:05 AM
Duration: 18m, 52s
Rows Produced: 5
Aggregate Peak Memory Usage: 125.2 MiB
Bytes Streamed: 17.0 KiB
Client Fetch Wait Time: 8ms
Client Fetch Wait Time Percentage: 0
Estimated per Node Peak Memory: 256.0 MiB
File Formats: PARQUET/NONE,PARQUET/SNAPPY
HDFS Average Scan Range: 3.5 KiB
HDFS Bytes Read: 304.9 KiB
HDFS Bytes Read From Cache: 0 B
HDFS Bytes Read From Cache Percentage: 0
HDFS Local Bytes Read: 304.9 KiB
HDFS Local Bytes Read Percentage: 100
HDFS Remote Bytes Read: 0 B
HDFS Remote Bytes Read Percentage: 0
HDFS Scanner Average Read Throughput: 84.2 MiB/s
HDFS Short Circuit Bytes Read: 304.9 KiB
HDFS Short Circuit Bytes Read Percentage: 100
Impala Version: impalad version 2.3.0-cdh5.5.2 RELEASE (build cc1125f10419a7269366f7f950f57b24b07acd64)
Memory Accrual: 612,880,244 byte seconds
Per Node Peak Memory Usage: 7.5 MiB
Planning Wait Time: 18.8m
Planning Wait Time Percentage: 100
Pool: root.default
Query Status: OK
Session ID: 83470f9951838902:61749525b39b70b7
Session Type: HIVESERVER2
Statistics Missing: true
Threads: CPU Time: 1.4m
Threads: CPU Time Percentage: 31
Threads: Network Receive Wait Time: 2.8m
Threads: Network Receive Wait Time Percentage: 64
Threads: Network Send Wait Time: 1.16s
Threads: Network Send Wait Time Percentage: 0
Threads: Storage Wait Time: 12.72s
Threads: Storage Wait Time Percentage: 5
Threads: Total Time: 4.4m
Work CPU Time: 1.4m