I have a table with goods:
CREATE TABLE public.goods (
"id" bigserial NOT NULL,
title varchar(250) NOT NULL,
cost numeric(10,2),
PRIMARY KEY ("id")
);
Now I want to sort this table by title but put all goods with cost 0 at the end of the list. Is this possible?
If I try to use:
ORDER BY
cost DESC,
title ASC
I get incorrect order by title
One way to do this is to use a CASE expression when ordering which places the block of records having a zero cost at the bottom. Then, within each block (either zero cost or non-zero cost), the records can be sorted alphabetically by the title.
SELECT cost, title
FROM public.goods
ORDER BY CASE WHEN cost = 0 THEN 1 ELSE 0 END,
title
Related
I have 2 tables in my postgresql timescaledb database (version 12.06) that I try to query through inner join.
Tables' structure:
CREATE TABLE currency(
id serial PRIMARY KEY,
symbol TEXT NOT NULL,
name TEXT NOT NULL,
quote_asset TEXT
);
CREATE TABLE currency_price (
currency_id integer NOT NULL,
dt timestamp WITHOUT time ZONE NOT NULL,
open NUMERIC NOT NULL,
high NUMERIC NOT NULL,
low NUMERIC NOT NULL,
close NUMERIC,
volume NUMERIC NOT NULL,
PRIMARY KEY (
currency_id,
dt
),
CONSTRAINT fk_currency FOREIGN KEY (currency_id) REFERENCES currency(id)
);
The query I'm trying to make is:
SELECT currency_id AS id, symbol, MAX(close) AS close, DATE(dt) AS date
FROM currency_price
JOIN currency ON
currency.id = currency_price.currency_id
GROUP BY currency_id, symbol, date
LIMIT 100;
Basically, it returns all the rows that exist in currency_price table. I know that postgres doesn't allow select columns without an aggregate function or including them in "group by" clause. So, if I don't include dt column in my select query, i receive expected results, but if I include it, the output shows rows of every single day of each currency while I only want to have the max value of every currency and filter them out based on various dates afterwards.
I'm very inexperienced with SQL in general.
Any suggestions to solve this would be very appreciated.
There are several ways to do it, easiest one comes to mind is using window functions.
select *
from (
SELECT currency_id,symbol,close,dt
,row_number() over(partition by currency_id,symbol
order by close desc,dt desc) as rr
FROM currency_price
JOIN currency ON currency.id = currency_price.currency_id
where dt::date = '2021-06-07'
)q1
where rr=1
General window functions:
https://www.postgresql.org/docs/9.5/functions-window.html
works also with standard aggregate functions like SUM,AVG,MAX,MIN and others.
Some examples: https://www.postgresqltutorial.com/postgresql-window-function/
I have different queries for fetching data from a large table (about 100-200M rows). I've created partial indexes for my table with different predicates to fit the query because I know each query.
For example, the table similar to this:
CREATE TABLE public.contacts (
id int8 NOT NULL DEFAULT ssng_generate_id(8::bigint),
created timestamp NOT NULL DEFAULT timezone('UTC'::text, now()),
contact_pool_id int8 NOT NULL,
project_id int8 NOT NULL,
state_id int4 NOT NULL DEFAULT 10,
order_x int4 NOT NULL,
next_attempt_date timestamp NULL,
CONSTRAINT contacts_pkey PRIMARY KEY (id)
);
And there are two types of query:
SELECT * FROM contacts WHERE contact_pool_id = X AND state_id = 10 ORDER BY order_x LIMIT 1;
and
SELECT * FROM contacts WHERE contact_pool_id = X AND state_id = 20 AND next_attemp_date <= NOW ORDER BY next_attemp_date LIMIT 1;
For those queries I've created partial indexes:
For state_id = 10 (new contacts)
CREATE INDEX ix_contacts_cpid_orderx_id_for_new ON contacts USING btree (contact_pool_id, order_x, id) WHERE state_id = 10;
For state_id = 20 (available contacts)
CREATE INDEX ix_contacts_cpid_nextattepmdate_id_for_available ON contacts USING btree (contact_pool_id, next_attempt_date, id) WHERE state_id = 20;
For me, those partial indexes are faster than a single index.
And what about an update and insert performance? If I change a row with state_id = 20, will it affect only index 2 (for available contacts) or both of them will be affected?
Partial indexes which are not relevant to the tuple will not get updated.
If PostgreSQL can do a HOT update (if the column being changed is not part of an index, and there is room on the same page for the new tuple), then even the relevant index doesn't need to get updated.
Yes, with a partial index you only pay the overhead of modifying the index for rows that meet the WHERE condition, so you will always only need to modify at most one of the indexes at the same time (unless you change state_id from 10 to 20 or vice versa).
I have a table consisting of products (with ID's, ~15k records) and another table price_changes (~88m records) recording a change in the price for a given productID at a given changedate.
I'm now interested in the price for each product at given points in time (say every 2 hours for a year, so altogether ~ 4300 points; altogether resulting in ~64m data points of interest). While it's very straight forward to determine the price for a given product at a given time, it seems to be quite time-consuming to determine all 64m data points.
My approach is to pre-populate a new target table fullprices with the data points of interest:
insert into fullprices(obsdate,productID)
select obsdate, productID from targetdates, products
and then update each price observation in this new table like this:
update fullprices f set price = (select price from price_changes where
productID = f.productID and date < f.obsdate
order by date desc
limit 1)
which should give me the most recent price change in each point in time.
Unfortunately, this takes ... well, ages. Is there any better way to do it?
== Edit: My tables are created as follows: ==
CREATE TABLE products
(
productID uuid NOT NULL,
name text NOT NULL,
CONSTRAINT products_pkey PRIMARY KEY (productID )
);
CREATE TABLE price_changes
(
id integer NOT NULL,
productID uuid NOT NULL,
price smallint,
date timestamp NOT NULL
);
CREATE INDEX idx_pc_date
ON price_changes USING btree
(date);
CREATE INDEX idx_pc_productID
ON price_changes USING btree
(productID);
CREATE TABLE targetdates
(
obsdate timestamp
);
CREATE TABLE fullprices
(
obsdate timestamp NOT NULL,
productID uuid NOT NULL,
price smallint
);
I have a date column which I want to be unique once populated, but want the date field to be ignored if it is not populated.
In MySQL the way this is accomplished is to set the date column to "not null" and give it a default value of '0000-00-00' - this allows all other fields in the unique index to be "checked" even if the date column is not populated yet.
This does not work in PosgreSQL because '0000-00-00' is not a valid date, so you cannot store it in a date field (this makes sense to me).
At first glance, leaving the field nullable seemed like an option, but this creates a problem:
=> create table uniq_test(NUMBER bigint not null, date DATE, UNIQUE(number, date));
CREATE TABLE
=> insert into uniq_test(number) values(1);
INSERT 0 1
=> insert into uniq_test(number) values(1);
INSERT 0 1
=> insert into uniq_test(number) values(1);
INSERT 0 1
=> insert into uniq_test(number) values(1);
INSERT 0 1
=> select * from uniq_test;
number | date
--------+------
1 |
1 |
1 |
1 |
(4 rows)
NULL apparently "isn't equal to itself" and so it does not count towards constraints.
If I add an additional unique constraint only on the number field, it checks only number and not date and so I cannot have two numbers with different dates.
I could select a default date that is a 'valid date' (but outside working scope) to get around this, and could (in fact) get away with that for the current project, but there are actually cases I might be encountering in the next few years where it will not in fact be evident that the date is a non-real date just because it is "a long time ago" or "in the future."
The advantage the '0000-00-00' mechanic had for me was precisely that this date isn't real and therefore indicated a non-populated entry (where 'non-populated' was a valid uniqueness attribute). When I look around for solutions to this on the internet, most of what I find is "just use NULL" and "storing zeros is stupid."
TL;DR
Is there a PostgreSQL best practice for needing to include "not populated" as a possible value in a unique constraint including a date field?
Not clear what you want. This is my guess:
create table uniq_test (number bigint not null, date date);
create unique index i1 on uniq_test (number, date)
where date is not null;
create unique index i2 on uniq_test (number)
where date is null;
There will be an unique constraint for not null dates and another one for null dates effectively turning the (number, date) tuples into distinct values.
Check partial index
It's not a best practice, but you can do it such way:
t=# create table so35(i int, d date);
CREATE TABLE
t=# create unique index i35 on so35(i, coalesce(d,'-infinity'));
CREATE INDEX
t=# insert into so35 (i) select 1;
INSERT 0 1
t=# insert into so35 (i) select 2;
INSERT 0 1
t=# insert into so35 (i) select 2;
ERROR: duplicate key value violates unique constraint "i35"
DETAIL: Key (i, (COALESCE(d, '-infinity'::date)))=(2, -infinity) already exists.
STATEMENT: insert into so35 (i) select 2;
I'm using postgresql 9.0 beta 4.
After inserting a lot of data into a partitioned table, i found a weird thing. When I query the table, i can see an empty row with null-like values in 'not-null' fields.
That weird query result is like below.
689th row is empty. The first 3 fields, (stid, d, ticker), are composing primary key. So they should not be null. The query i used is this.
select * from st_daily2 where stid=267408 order by d
I can even do the group by on this data.
select stid, date_trunc('month', d) ym, count(*) from st_daily2
where stid=267408 group by stid, date_trunc('month', d)
The 'group by' results still has the empty row.
The 1st row is empty.
But if i query where 'stid' or 'd' is null, then it returns nothing.
Is this a bug of postgresql 9b4? Or some data corruption?
EDIT :
I added my table definition.
CREATE TABLE st_daily
(
stid integer NOT NULL,
d date NOT NULL,
ticker character varying(15) NOT NULL,
mp integer NOT NULL,
settlep double precision NOT NULL,
prft integer NOT NULL,
atr20 double precision NOT NULL,
upd timestamp with time zone,
ntrds double precision
)
WITH (
OIDS=FALSE
);
CREATE TABLE st_daily2
(
CONSTRAINT st_daily2_pk PRIMARY KEY (stid, d, ticker),
CONSTRAINT st_daily2_strgs_fk FOREIGN KEY (stid)
REFERENCES strgs (stid) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE,
CONSTRAINT st_daily2_ck CHECK (stid >= 200000 AND stid < 300000)
)
INHERITS (st_daily)
WITH (
OIDS=FALSE
);
The data in this table is simulation results. Multithreaded multiple simulation engines written in c# insert data into the database using Npgsql.
psql also shows the empty row.
You'd better leave a posting at http://www.postgresql.org/support/submitbug
Some questions:
Could you show use the table
definitions and constraints for the
partions?
How did you load your data?
You get the same result when using
another tool, like psql?
The answer to your problem may very well lie in your first sentence:
I'm using postgresql 9.0 beta 4.
Why would you do that? Upgrade to a stable release. Preferably the latest point-release of the current version.
This is 9.1.4 as of today.
I got to the same point: "what in the heck is that blank value?"
No, it's not a NULL, it's a -infinity.
To filter for such a row use:
WHERE
case when mytestcolumn = '-infinity'::timestamp or
mytestcolumn = 'infinity'::timestamp
then NULL else mytestcolumn end IS NULL
instead of:
WHERE mytestcolumn IS NULL