How to simplify nested SQL cross join? - sql

Question

I'm using Postgres 9.3 and have the following four tables to have maximum flexibility regarding price and / or tax / taxe rate changes in the future (see below for more details):
CREATE TABLE main.products
(
id serial NOT NULL,
"productName" character varying(255) NOT NULL,
"productStockAmount" real NOT NULL,
)
CREATE TABLE main."productPrices"
(
id serial NOT NULL,
product_id integer NOT NULL,
"productPriceValue" real NOT NULL,
"productPriceValidFrom" timestamp without time zone NOT NULL,
)
CREATE TABLE main."productTaxes"
(
id serial NOT NULL,
product_id integer NOT NULL,
"productTaxValidFrom" timestamp without time zone NOT NULL,
"taxRate_id" integer NOT NULL,
)
CREATE TABLE main."taxRateValues"
(
id integer NOT NULL,
"taxRate_id" integer NOT NULL,
"taxRateValueValidFrom" timestamp without time zone NOT NULL,
"taxRateValue" real,
)
I built a view based on the following query to get the currently relevant values:
SELECT p.id, p."productName", p."productStockAmount", sub."productPriceValue", CHR(64+sub3."taxRate_id") AS taxRateId, sub3."taxRateValue" FROM main."products" p
CROSS JOIN LATERAL (SELECT * FROM main."productPrices" pp2 WHERE pp2."product_id"=p."id" AND pp2."productPriceValidFrom" <= NOW() ORDER BY pp2."productPriceValidFrom" DESC LIMIT 1) AS sub
CROSS JOIN LATERAL (SELECT * FROM main."productTaxes" pt WHERE pt."product_id"=p."id" AND pt."productTaxValidFrom" <= NOW() ORDER BY pt."productTaxValidFrom" DESC LIMIT 1) AS sub2
CROSS JOIN LATERAL (SELECT * FROM main."taxRateValues" trv WHERE trv."taxRate_id"=sub2."taxRate_id" AND trv."taxRateValueValidFrom" <= NOW() ORDER BY trv."taxRateValueValidFrom" DESC LIMIT 1) AS sub3
This works fine and gives me the correct results but I assume to get performance problems if several thousand products, price changes etc. are in the database.
Is there anything I can do to simplify the statement or the overall database design?
To use words to describe the needed flexibility:
Prices can be changed and I have to record which price is valid to which time (archival, so not only the current price is needed)
Applied tax rates for products can be changed (e.g. due to changes by law) - archival also needed
Tax rates in general can be changed (also by law, but not related to a single product but all products with this identifier)
Some examples of things that can happen:
Product X changes price from 100 to 200 at 2014-05-09
Product X changes tax rate from A to B at 2014-07-01
Tax rate value for tax rate A changes from 16 to 19 at 2014-09-01

Answer 1

As long as you fetch all rows or more than a few percent of all rows, it will be substantially faster to first aggregate once per table, and then join.
I suggest DISTINCT ON to pick the latest valid row per id:
SELECT p.id, p."productName", p."productStockAmount"
,pp."productPriceValue"
,CHR(64 + tr."taxRate_id") AS "taxRateId", tr."taxRateValue"
FROM main.products p
LEFT JOIN (
SELECT DISTINCT ON (product_id)
product_id, "productPriceValue"
FROM main."productPrices"
WHERE "productPriceValidFrom" <= now()
ORDER BY product_id, "productPriceValidFrom" DESC
) pp ON pp.product_id = p.id
LEFT JOIN (
SELECT DISTINCT ON (product_id)
product_id, "taxRate_id"
FROM main."productTaxes"
WHERE "productTaxValidFrom" <= now()
ORDER BY product_id, "productTaxValidFrom" DESC
) pt ON pt.product_id = p.id
LEFT JOIN (
SELECT DISTINCT ON ("taxRate_id") *
FROM main."taxRateValues"
WHERE "taxRateValueValidFrom" <= now()
ORDER BY "taxRate_id", "taxRateValueValidFrom" DESC
) tr ON tr."taxRate_id" = pt."taxRate_id";
Using LEFT JOIN to be on the safe side. Not every product might have entries in all sub-tables.
And I subscribe to what #Clodoaldo wrote about double-quoted identifiers. I never use anything but legal, lower-case names. Makes your life with Postgres easier.
Detailed explanation for DISTINCT ON:
Select first row in each GROUP BY group?

Answer 2

Do not create quoted identifiers. Once you do it you are forever stuck with them and you will have to quote and remember the casing everywhere. You can use camel case whenever you want if you don't quote the identifier at creation time.
I don't understand why you need the cross lateral. I think it can be just
select
p.id,
p."productName",
p."productStockAmount",
pp2."productPriceValue",
chr(64 + trv."taxRate_id") as "taxRateId",
trv."taxRateValue"
from
main."products" p
left join (
select *
from main."productPrices"
where "productPriceValidFrom" <= now()
order by "productPriceValidFrom" desc
limit 1
) pp2 on pp2."product_id" = p."id"
left join (
select "taxRate_id"
from main."productTaxes"
where "productTaxValidFrom" <= now()
order by "productTaxValidFrom" desc
limit 1
) pt on pt."product_id" = p."id"
left join (
select *
from main."taxRateValues"
where "taxRateValueValidFrom" <= now()
order by "taxRateValueValidFrom" desc
limit 1
) trv on trv."taxRate_id" = pt."taxRate_id"