What is the difference between CURRENT_TIMESTAMP and clock_timestamp()
CURRENT_TIMESTAMP is an ANSI-SQL Standard variable you will find in many relational databases including PostgreSQL, SQL Server, Firebird, IBM DB2 and MySQL to name a few
that records the start of the transaction. The important thing to keep in mind about it is there is only one entry per transaction so if you have a long running transaction,
you won't be seeing it changing as you go along.
clock_timestamp() is a PostgreSQL function that always returns the current clock's timestamp. I don't think I'm alone in using it for doing simple benchmarking and other things
where for example I need to record the timings of each part of a function within the function using pedestrian RAISE NOTICE debug print statements.
There is another cool way I like using it, and that is for a batch of records each with an expensive function call, benchmarking how long it takes to process each record.
One of the things I'm working on is improving the speed of the tiger_geocoder packaged in PostGIS 2.0. The first root of attack seemed to me would be the normalize_address function
which I was noticing was taking anywhere from 10% to 50% of my time in the geocode process. That's a ton of time if you are trying to batch geocode a ton of records. The thing is
the function is very particular to how badly formed the address is so a whole batch could be held up by one bad apple and since the batch doesn't return until all are processed, it makes
the whole thing seem to take a while.
So rather than looping thru each, I thought it would be cool if I could run the batch, but for each record have it tell me how long it took to process relative to the rest so I could get
a sense of what a problem address looks like. So I wrote this query:
the_time - COALESCE(lag(the_time) OVER(ORDER BY the_time), CURRENT_TIMESTAMP) As process_time,
the_time - CURRENT_TIMESTAMP As diff_from_start
FROM (SELECT address_1, city, state, zip,
pprint_addy(normalize_address(coalesce(address_1,'') || ', ' || coalesce(city || ' ','') || state || ' ' || zip)) As pp_addr,
clock_timestamp() As the_time
FROM testgeocode LIMIT 1000) As foo )
WHERE process_time > '00:00:00.016'::interval;
Which returned an output something like this:
address_1 | city | state | zip | pp_addr | the_time | process_time | diff_from_start
48 MAIN ST .. | S.. | MA | 021.. | 48 MAIN .. | 2011-05-10 03:24:43.078-04 | 00:00:00.032 | 00:00:00.032
15 ... | | MA | 018... | 15 GREN... | 2011-05-10 03:24:50.796-04 | 00:00:00.031 | 00:00:07.75