Hello,
I have a general query optimization question involving repeated calculations.
I frequently want to make views that repeat a calculation over and over again in lots of columns. In the example below, let’s say it’s (a+b), but we can imagine the calculation being much more complicated.
For example:
CREATE VIEW AS
SELECT (a+b)*c as c1, (a+b)*d as d1, (a+b)*e as e1
FROM table;
My question is, is PostgreSQL "smart" enough to cache this (a+b) calculation in the above example, or does it recalculate it in each column? If it recalculates it in each column, would I generally get better performance by using a subquery to convince PostgreSQL to cache the result? For example:
CREATE VIEW AS
SELECT x*c as c1, x*d as d1, x*e as e1
FROM (SELECT (a+b) as x, * FROM table) x;
Or perhaps I could use a CTE? In some instances, I might need 2 or more subqueries to “cache” all the calculations (if one column depends on another column that depends on another column), and I feel that sometimes Postgres makes better optimization decisions when there are not subqueries involved, but I am not sure.
Obviously I could benchmark everything every single time this comes up, but there is overhead in that and some kind of rule of thumb or general guideline would be helpful as it is a frequent question for me.
Thank you!
Robert
I have a general query optimization question involving repeated calculations.
I frequently want to make views that repeat a calculation over and over again in lots of columns. In the example below, let’s say it’s (a+b), but we can imagine the calculation being much more complicated.
For example:
CREATE VIEW AS
SELECT (a+b)*c as c1, (a+b)*d as d1, (a+b)*e as e1
FROM table;
My question is, is PostgreSQL "smart" enough to cache this (a+b) calculation in the above example, or does it recalculate it in each column? If it recalculates it in each column, would I generally get better performance by using a subquery to convince PostgreSQL to cache the result? For example:
CREATE VIEW AS
SELECT x*c as c1, x*d as d1, x*e as e1
FROM (SELECT (a+b) as x, * FROM table) x;
Or perhaps I could use a CTE? In some instances, I might need 2 or more subqueries to “cache” all the calculations (if one column depends on another column that depends on another column), and I feel that sometimes Postgres makes better optimization decisions when there are not subqueries involved, but I am not sure.
Obviously I could benchmark everything every single time this comes up, but there is overhead in that and some kind of rule of thumb or general guideline would be helpful as it is a frequent question for me.
Thank you!
Robert
