HINT: Perhaps you need a different "datestyle" setting.Ĭonclusion: Converting a column from one data type to another is nothing you want to do when you system is live. In session 1 you’ll notice that the statement is blocked and finally you get this: Time: 0.453 msĮRROR: date/time field value out of range: "01012017" In session 2 I am doing the conversion: Time: 0.453 > alter table t1 alter column a type date using to_date(a,'DDMMYYYY') Then, in session 1 I am doing this: Time: 0.453 > select count(*) from t1 where a = '01012017' and so on and so on > insert into t1 select * from t1 Insert into t1 (a) values > insert into t1 select * from t1 The remaining question is: Does this operation block others from reading the table while it is being executed? Lets generate some more date to make the select operation a bit longer and then “watch” the statement in a separate session while we execute the conversion: Time: 0.453 ms Time: 20.972 > explain analyze select a from t1 where a = to_date('01012017','dd.mm.yyyy') Time: 3.344 > insert into t1 select * from t1 Time: 1.432 > insert into t1 select * from t1 Time: 1.455 > insert into t1 select * from t1 Time: 1.072 > insert into t1 select * from t1 Time: 1.110 > insert into t1 select * from t1
Time: 39.653 > insert into t1 select * from t1 Time: 2.373 > insert into t1 select * from t1 Looks fine as well, lets do a quick test if the index is really usable: Time: 0.453 > insert into t1 select * from t1 "i1" btree > select indisvalid,indisready,indislive from pg_index where indexrelid = 'i1'::regclass "i1" btree > alter table t1 alter column a type date using to_date(a,'DDMMYYYY') When there is an index on the column, what happens to the index? > d t1 That’s really cool: You can pass a function (this mean a self written function as well) to the alter command to specify on how you want to do the conversion. What will really help here is the “using” keyword because you can do things like this: > alter table t1 alter column a type date using to_date(a,'DDMMYYYY') When you look at the “alter table” command in PostgtreSQL you’ll notice something like this: where action is one of:ĪDD column_name data_type ]ĭROP column_name ĪLTER column_name TYPE data_type We now have a varchar column holding the date values as a string. Most of the alternative names listed in the Aliases column are the names used internally by PostgreSQL for historical reasons.
Table 8.1 shows all the built-in general-purpose data types. Users can add new types to PostgreSQL using the CREATE TYPE command. Insert into t1 (a) values > select * from t1 PostgreSQL has a rich set of native data types available to users. To start with lets generate some test data: drop table if exists t1 This for sure will work but it is not the most convenient way: What happens if the application is somehow dependent on the order of the columns? Shouldn’t be the case if the application is written well, but you never know. How can we do that in PostgreSQL? The first answer that came to me mind is: Add a new column (of data type date) and populate it with the converted string from the source column, drop the source column and rename the new column to the name of the dropped column. When we want to convert this column to be a real date data type in Oracle we can use dbms_redefinition. It will give you the same result as you were expecting from using serial.Last week at a customer the following question came up: We have a varchar2 column in Oracle that contains a date but actually is stored as a string. The second line alters your table with the new default value, which will be determined by the previously created sequence. The START statement defines what value this sequence should start from. In your case the table is address and the column is new_id. OWNED BY statement connects the newly created sequence with the exact column of your table. The first line of the query creates your own sequence called my_serial.
#POSTGRES CHANGE COLUMN DATA TYPE SERIAL#
In case you would like to achieve the same effect, as you are expecting from using serial data type when you are altering existing table you may do this: CREATE SEQUENCE my_serial AS integer START 1 OWNED BY address.new_id ĪLTER TABLE address ALTER COLUMN new_id SET DEFAULT nextval('my_serial') Because serial is not a true data type, but merely an abbreviation or alias for a longer query. If you'll try to ALTER an existing table using this data type you'll get an error. This happened because you may use the serial data type only when you are creating a new table or adding a new column to a table.
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