Insert in HIVE External Table is very slow - hive

Question

I have a table, stop_logs in HIVE. When I run a insert query for around 6000 rows, it takes 300 secs, where as if I run just SELECT query, it finishes in 6 seconds. Why insert is taking this much time?
CREATE TABLE stop_logs (event STRING, loadId STRING)
STORED AS SEQUENCEFILE;
Following takes 300 sec:
INSERT INTO TABLE stop_logs
SELECT
i.event, i.loadId
FROM
event_logs i
WHERE
i.stopId IS NOT NULL;
;
Following query takes 6 secs.
SELECT
i.event, i.loadId
FROM
event_logs i
WHERE
i.stopId IS NOT NULL;
;

Answer 1

First you need to understand how Hive is processing your query:
When you perform a "select * from < tablename>", Hive fetches the whole data from file as a FetchTask rather than a mapreduce task which just dumps the data as it is without doing anything on it. This is similar to "hadoop dfs -text ". As it doesn't run any map-reduce task so it runs faster.
while using "select a,b from < tablename>", Hive requires a map-reduce job since it needs to extract the 'column' from each row by parsing it from the file it loads.
While using "insert into table stop_logs select a,b from event_logs" statement, first select statement runs, which trigger map-reduce job since it needs to extract the 'column' from each row by parsing it from the file it loads and for inserting into another table(stop_logs) it will launch another map reduce task to takes values inserted into columns a and b in 'stop_logs' and maps them to columns a and b, respectively, for insertion into the new row.
Another reason for slowness is check If "hive.typecheck.on.insert" is set to true ,because of that values are validated, converted and normalized to conform to their column types (Hive 0.12.0 onward) while inserting into table which also causes insert to perform slow as compare to select statement.