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Answer Posted / bikash khuntia
High watermark means, the largest amouth of data
that was placed in a table. for example, Let's say
you created a table and inserted 100 records, and then
added 1000 records more. Now the table highwater mark is
the 1000 record. Now let's say you deleted all the
records in that table using the 'delete' command. Now
your highwater mark for the table still sits at the 1000
record . Though after the delete you have no records
in the table, when a query is performed on this empty
table, it will still scan upto the 1000 highwater mark,
So even with no records, the query will still take
sometime returning because it scans up to the high
watermark.
Now, instead of deleating the records with delete command,
You trucate the tabel, Truncate will remove the high
watermark for the table, and now on an empty table the
query is faster due to the high watermark being gone
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