I have a table where part of the primary key is a foreign key to another table.
create table player_result (
event_id integer not null,
pub_time timestamp not null,
name_key varchar(128) not null,
email_address varchar(128),
withdrawn boolean not null,
place integer,
realized_values hstore,
primary key (event_id, pub_time, name_key),
foreign key (email_address) references email(address),
foreign key (event_id, pub_time) references event_publish(event_id, pub_time));
Will the index generated for the primary key suffice to back the foreign key on event_id and pub_time?
Yes.
Index A,B,C
is good for:
A
A,B
A,B,C (and any other combination of the full 3 fields, if default order is unimportant)
but not good for other combinations (such as B,C, C,A etc.).
It will be useful for the referencing side, such that a DELETE or UPDATE on the referenced table can use the PRIMARY KEY of the referencing side as an index when performing checks for the existence of referencing rows or running cascade update/deletes. PostgreSQL doesn't require this index to exist at all, it just makes foreign key constraint checks faster if it is there.
It is not sufficient to serve as the unique constraint for a reference to those columns. You couldn't create a FOREIGN KEY that REFERENCES player_result(event_id, pub_time) because there is no unique constraint on those columns. That pair can appear multiple times in the table so long as each pair has a different name_key.
As #xagyg accurately notes, the unique b-tree index created by the foreign key reference is also only useful for references to columns from the left of the index. It could not be used for a lookup of pub_time, name_key or just name_key, for example.
Related
in postgres, just checking if we need to index all columns of composite primary key
CREATE TABLE BOOK_TYPE(
ID TEXT NOT NULL,
TYPE TEXT NOT NULL,
LABELS HSTORE NOT NULL,
CONSTRAINT BOOK_TYPE_PKEY PRIMARY KEY (ID,TYPE)
);
should I have to index ID and type separately?
You don't need to create any extra index unless you happen to need it to speed up a query. The primary key will automatically create a unique index on (id, type), and that is all that is needed to guarantee consistency.
I'm trying to run create_distributed_table for tables which i need to shard and almost all of the tables have self relation ( parent child )
but when I run SELECT create_distributed_table('table-name','id');
it throws error cannot create foreign key constraint
simple steps to reproduce
CREATE TABLE TEST (
ID TEXT NOT NULL,
NAME CHARACTER VARYING(255) NOT NULL,
PARENT_ID TEXT
);
ALTER TABLE TEST ADD CONSTRAINT TEST_PK PRIMARY KEY (ID);
ALTER TABLE TEST ADD CONSTRAINT TEST_PARENT_FK FOREIGN KEY (PARENT_ID) REFERENCES TEST (ID);
ERROR
citus=> SELECT create_distributed_table('test','id');
ERROR: cannot create foreign key constraint
DETAIL: Foreign keys are supported in two cases, either in between two colocated tables including partition column in the same ordinal in the both tables or from distributed to reference tables
For the time being, it is not possible to shard a table on PostgreSQL without dropping the self referencing foreign key constraints, or altering them to include a separate and new distribution column.
Citus places records into shards based on the hash values of the distribution column values. It is most likely the case that the hashes of parent and child id values are different and hence the records should be stored in different shards, and possibly on different worker nodes. PostgreSQL does not have a mechanism to create foreign key constraints that reference records on different PostgreSQL clusters.
Consider adding a new column tenant_id and adding this column to the primary key and foreign key constraints.
CREATE TABLE TEST (
tenant_id INT NOT NULL,
id TEXT NOT NULL,
name CHARACTER VARYING(255) NOT NULL,
parent_id TEXT NOT NULL,
FOREIGN KEY (tenant_id, parent_id) REFERENCES test(tenant_id, id),
PRIMARY KEY (tenant_id, id)
);
SELECT create_distributed_table('test','tenant_id');
Note that parent and child should always be in the same tenant for this to work.
PostgreSQL/PGAdmin4 ERROR: there is no unique constraint matching given keys for referenced table
This is the ‘schema’ I’m trying to code into PGAdmin4/Postgresql:
http://i.imgur.com/xPEu8Sh.jpg
I was able to convert all tables, except “QUALIFICATIONS”.
I tried to process the following query:
create table REGISTRATION(
StudentID int,
SectionNo int,
Semester varchar(16),
foreign key(StudentID) references Student(StudentID),
foreign key (SectionNo, Semester) references Section(SectionNo, Semester),
primary key(studentID, SectionNo, Semester)
);
I received the following message:
ERROR: there is no unique constraint matching given keys for referenced table "section"
These are the foreign and primary I have in the table SECTION
PKEY: i.imgur.com/BcUNKug.jpg
FKEY: i.imgur.com/D8B8hRW.jpg
Code of SECTION table:
CREATE TABLE class_scheduling_01.section
(
sectionno integer NOT NULL,
semester character varying(16) COLLATE pg_catalog."default" NOT NULL,
courseid character varying(16) COLLATE pg_catalog."default" NOT NULL,
CONSTRAINT section_pkey PRIMARY KEY (sectionno, semester, courseid),
CONSTRAINT section_sectionno_key UNIQUE (sectionno),
CONSTRAINT section_courseid_fkey FOREIGN KEY (courseid)
REFERENCES class_scheduling_01.course (courseid) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION
)
I additionally ran the command: ALTER TABLE section ADD UNIQUE (sectionno);
Since none of the attributes seemed to be repeating itself.
Despite all this I’m getting:
ERROR: there is no unique constraint matching given keys for referenced table "section" Query returned successfully in 642 msec.
Edit: I've gone back to the COURSE table and made courseID a unique constraint. I still get the same message. SECTION table has a composite primary key made up of 3 columns. As seen in the first picture linked, out of all the values, only SECTION.sectionno is the only column with unique/non-repeating values.
2nd edit: I decided to create the table "REGISTRATION" one step at a time, and make the foreign keys last with alter table command.
I was able to make the columns StudentID andd SectionNo foreign keys to their respective columns. When I tried to make REGISTRATION.semester a foreign key to SECTION.semester I got the error message again.
alter table REGISTRATION add foreign key (semester) references section(semester);
As seen in the image I linked Semester value, is repeated; despite this, am I still required to make it unique? Or do I make a unique command assigning all 3 columns (of SECTION) together as unique, instead of just 1? If so, how?
This
foreign key (SectionNo, Semester) references Section(SectionNo, Semester),
requires that there be a unique constraint on the pair of columns SectionNo and Semester.
CREATE TABLE class_scheduling_01.section
(
sectionno integer NOT NULL,
semester character varying(16) COLLATE pg_catalog."default" NOT NULL,
courseid character varying(16) COLLATE pg_catalog."default" NOT NULL,
CONSTRAINT section_pkey PRIMARY KEY (sectionno, semester, courseid),
CONSTRAINT section_sectionno_key UNIQUE (sectionno),
CONSTRAINT section_courseid_fkey FOREIGN KEY (courseid)
REFERENCES class_scheduling_01.course (courseid) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION,
-- Unique constraint on the pair
CONSTRAINT your_constraint_name UNIQUE (SectionNo, Semester)
);
That change should let these SQL statements succeed. I didn't check to see whether that's a good idea.
SQL is case insensitive.
I understand what you mean, but this is a bad way to think about it.
PostgreSQL folds unquoted identifiers to lowercase. So PostgreSQL would treat the identifiers SQL, Sql, and sql as if they were all sql. A quoted or delimited identifier, like "Select" always refers to either a table or a column; it's never interpreted as a keyword. Quoted identifiers are case-sensitive. You can't successfully refer to the table "Select" as select.
I'm not sure which indexes postgresql AUTOMATICALLY creates; I think it will create one on Book(ibsn), because its the primary key, and also Book(title), because its a candidate key... but I'm not sure if postgres will automatically create a combined index on all key attributes Book(ibsn, title). Also, would it create any other indexes automatically?
CREATE TABLE Book (
isbn INTEGER CONSTRAINT B_ISBN CHECK (ISBN BETWEEN 1 AND 2000),
title VARCHAR(200) CONSTRAINT B_TITLE NOT NULL UNIQUE,
author VARCHAR(50) CONSTRAINT B_AUTH NOT NULL,
cost FLOAT DEFAULT 0.00,
lent_date DATE,
returnDate DATE,
times_lent INTEGER,
sectionID SMALLINT,
CONSTRAINT BOOK_PRIME PRIMARY KEY (isbn),
CONSTRAINT BOOK_SECT FOREIGN KEY (sectionID) REFERENCES Section(sectionID) ON DELETE CASCADE
);
Postgres will automatically create indexes only for:
primary keys: Adding a primary key will automatically create a unique B-tree index on the column or group of columns listed in the primary key
unique constraints: Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint
In your case, Postgres creates one unique index on the column isbn and one unique index on the column unique because you declared each column individually to be unique, not the combination of both.
No other indexes will be created automatically.
I have made classical Employee - Employer table that has one primary key and one foreign key. Foreign key references primary so it is a self referencing table:
CREATE TABLE Worker
(
OIB NUMERIC(2,0),
Name NVARCHAR(10),
Surname NVARCHAR(20),
DateOfEmployment DATETIME2 NOT NULL,
Adress NVARCHAR(20),
City NVARCHAR(10),
SUPERIOR NUMERIC(2,0) UNIQUE,
Constraint PK_Worker PRIMARY KEY(OIB),
CONSTRAINT FK_Worker FOREIGN KEY (Superior) REFERENCES Worker(OIB)
);
Second table that should keep points for all my employees is made like this:
CREATE TABLE Point
(
OIB_Worker NUMERIC(2,0) NOT NULL,
OIB_Superior NUMERIC(2,0) NOT NULL,
Pt_To_Worker tinyint,
Pt_To_Superior tinyint,
Month_ INT NOT NULL,
Year_ INT NOT NULL,
CONSTRAINT FK_Point_Worker FOREIGN KEY (OIB_Worker) References Worker(OIB),
CONSTRAINT FK_Point_Worker_2 FOREIGN KEY (OIB_Superior) References Worker(Superior),
CONSTRAINT PK_Point PRIMARY KEY(OIB_Worker,OIB_Superior,Month_,Year_)
);
It should enable storing grades per month for every employee.
That is, I have two foreign keys, one for Worker.OIB and one for Worker.Superior. Also, I have a composite primary key made of columns Point.OIB_Worker, Point.OIB_superior, Point.Month_ and Point.Year_. Key is composite because it needs to disable entering grades more then once a month.
My question is:
How to make a foreign key from Point to Worker so that any superior can have more then one employee assigned to him?
If you look closely, my implementation works but it can have only one employee per manager.
That is because of a fact that a foreign key has to reference either the primary or the unique column from other table. And my Worker.Superior is UNIQUE, so it can have only unique values (no repetition).
I think many people will find this example interesting as it is a common problem when making a new database.
I think your FK_Point_Worker_2 should also have References Worker(OIB), and you should remove the UNIQUE constraint from Worker.Superior. That way a superior can have more than one worker assigned to him.
Think about it. You have unique constraint on SUPERIOR and you are confused as to why two employees cannot have the same SUPERIOR. That is what a unique constraint does - not allow duplicates.
A FK can only reference a unique column or columns.
A FK_Point_Worker_2 with a References Worker(OIB) does not assure OIB is a Superior.
I would add a unique constraint on Worker on (OIB, SUPERIOR)
and remove the unique constraint on SUPERIOR.
It will always be unique as OIB is unique.
Then have composite FK relationship.
This is an example of a composite FK relationship
ALTER TABLE [dbo].[wfBchFolder] WITH CHECK ADD CONSTRAINT [FK_wfBchFolder_wfBch] FOREIGN KEY([wfID], [bchID])
REFERENCES [dbo].[WFbch] ([wfID], [ID])
GO