Computed / calculated / virtual / derived columns

问题:

Does PostgreSQL support computed / calculated columns, like MS SQL Server? I can\'t find anything in the docs, but as this feature is included in many other DBMSs I thought I might be missing something.

Eg: http://msdn.microsoft.com/en-us/library/ms191250.aspx

回答1:

Up to Postgres 11 generated columns are not supported - as defined in the SQL standard and implemented by some RDBMS including DB2, MySQL and Oracle. Nor the similar \"computed columns\" of SQL Server.

The feature is in development for Postgres 12, principle author Peter Eisentraut.

For now, you can emulate with a function using attribute notation (tbl.col) that looks and works much like a virtual generated column. That\'s a bit of a syntax oddity which exists in Postgres for historic reasons and happens to fit the case. This related answer has code examples:

• Store common query as column?

The expression (looking like a column) is not included in a SELECT * FROM tbl, though. You always have to list it explicitly.

Can also be supported with a matching expression index - provided the function is IMMUTABLE. Like:

CREATE FUNCTION col(tbl) ... AS ...  -- your computed expression here
CREATE INDEX ON tbl(col(tbl));

Alternatives

Alternatively, you can implement similar functionality with a VIEW, optionally coupled with expression indexes. Then SELECT * can include the generated column.

\"Persisted\" computed columns can be implemented with triggers in a functionally identical way.

Materialized views are a closely related concept, implemented since Postgres 9.3.
In earlier versions one can manage MVs manually.

回答2:

YES you can!! The solution should be easy, safe, and performant...

I\'m new to postgresql, but it seems you can create computed columns by using an expression index, paired with a view (the view is optional, but makes makes life a bit easier).

Suppose my computation is md5(some_string_field), then I create the index as:

CREATE INDEX some_string_field_md5_index ON some_table(MD5(some_string_field));

Now, any queries that act on MD5(some_string_field) will use the index rather than computing it from scratch. For example:

SELECT MAX(some_field) FROM some_table GROUP BY MD5(some_string_field);

You can check this with explain.

However at this point you are relying on users of the table knowing exactly how to construct the column. To make life easier, you can create a VIEW onto an augmented version of the original table, adding in the computed value as a new column:

CREATE VIEW some_table_augmented AS 
   SELECT *, MD5(some_string_field) as some_string_field_md5 from some_table;

Now any queries using some_table_augmented will be able to use some_string_field_md5 without worrying about how it works..they just get good performance. The view doesn\'t copy any data from the original table, so it is good memory-wise as well as performance-wise. Note however that you can\'t update/insert into a view, only into the source table, but if you really want, I believe you can redirect inserts and updates to the source table using rules (I could be wrong on that last point as I\'ve never tried it myself).

Edit: it seems if the query involves competing indices, the planner engine may sometimes not use the expression-index at all. The choice seems to be data dependant.

回答3:

One way to do this is with a trigger!

CREATE TABLE computed(
    one SERIAL,
    two INT NOT NULL
);

CREATE OR REPLACE FUNCTION computed_two_trg()
RETURNS trigger
LANGUAGE plpgsql
SECURITY DEFINER
AS $BODY$
BEGIN
    NEW.two = NEW.one * 2;

    RETURN NEW;
END
$BODY$;

CREATE TRIGGER computed_500
BEFORE INSERT OR UPDATE
ON computed
FOR EACH ROW
EXECUTE PROCEDURE computed_two_trg();

The trigger is fired before the row is updated or inserted. It changes the field that we want to compute of NEW record and then it returns that record.

回答4:

I have a code that works and use the term calculated, I\'m not on postgresSQL pure tho we run on PADB

here is how it\'s used

create table some_table as
    select  category, 
            txn_type,
            indiv_id, 
            accum_trip_flag,
            max(first_true_origin) as true_origin,
            max(first_true_dest ) as true_destination,
            max(id) as id,
            count(id) as tkts_cnt,
            (case when calculated tkts_cnt=1 then 1 else 0 end) as one_way
    from some_rando_table
    group by 1,2,3,4    ;

回答5:

Well, not sure if this is what You mean but Posgres normally support \"dummy\" ETL syntax. I created one empty column in table and then needed to fill it by calculated records depending on values in row.

UPDATE table01
SET column03 = column01*column02; /*e.g. for multiplication of 2 values*/

1. It is so dummy I suspect it is not what You are looking for.
2. Obviously it is not dynamic, you run it once. But no obstacle to get it into trigger.

回答6:

A lightweight solution with Check constraint:

CREATE TABLE example (
    discriminator INTEGER DEFAULT 0 NOT NULL CHECK (discriminator = 0)
);