However, ‘NOW’::timestamptz is a constant. This means that the default value inserted will change over time as transactions are started and committed. +-+-+-+-įield | timestamp with time zone | | | ' 10:49:06.741606+02'::timestamp with time zoneįield | timestamp with time zone | | | now()Īs I said before: now() is a function and therefore PostgreSQL will use the function call as the default value for the column. The following example shows why:Ĭolumn | Type | Collation | Nullable | Default In this case it makes all the difference in the world. Test=# CREATE TABLE b (field timestamptz DEFAULT now()) Test=# CREATE TABLE a (field timestamptz DEFAULT 'NOW'::timestamptz) What if we want timestamps in our table definition? Using ‘NOW’::timestamptz in table definitions 10:56:21.310924+02 | 10:56:21.310924+02Īs expected both flavors of “now” will return transaction time which means that the time within the very same transaction will stay the same. Why is that relevant? At first glance it seems to make no difference: My description already contains the magic words “function” and “constant”. Most people are not aware of the fact that there is actually a difference between now() as a function and ‘NOW’::timestamptz as a constant. To make it easier for beginners, as well as advanced people, to understand this vital topic I have decided to compile some examples which are important for your everyday work. However, in PostgreSQL there are some subtle issues most people might not be aware of. This method works in Netezza as well.Everybody who has ever written any kind of database application had to use time and date. This can also be used to calculate quarters or fiscal quarters or whatever… but that’s a question for another post.Using a string like the following will push the date or timestamp to the start of the month. It helps to use DATE_TRUNC on the dates you are using to standardize them if you don’t need to be uber-precise.You can then cast this as whatever you’d like, do additional calculations, etc. In the above example, this will yield 14. EXTRACT the number of months from the interval.Multiply the number of years by 12 to get the number of months counted by those years (seems obvious, but I am outlining all the steps in excruciating detail).EXTRACT the number of years from the interval.This will return an interval that may look something like “1 year 2 months 3 days” Note that it is subtracting the second from the first, so you must have them in this order. Use the AGE function to calculate the difference between the end date and start date.EXTRACT(year FROM age(end_date,start_date))*12 + EXTRACT(month FROM age(end_date,start_date)) You can then EXTRACT the pieces of the interval needed to calculate months and use them in a simple equation. This may not seem like we’ve gotten very far in calculating the number of months between the two dates, but stick with me on this one. The age function calculates the difference between two dates and returns an interval. The best way I have found to get around this is to use the built in AGE function. In PostgreSQL, subtracting one date from another will yield a number of days that then takes a tremendously complex formula to manipulate into months. Often it is more helpful to show the date as a number of months rather than a number of days. One of the most basic calculations on dates is to tell the time elapsed between two dates.
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