What is the difference between primary key constraint vs primary key sequence when it comes to postgres db?
A primary key constraint constrains (restricts) values in the column to being unique.
A primary key sequence defines the way new unique values for a primary key are generated.
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 using PostgreSQL.
I have a table accounts with account_id as the primary key. I also have a second table called relations with a composite primary key (follower_id, following_id). Each relation must be unique.
ALTER TABLE accounts ADD CONSTRAINT users_pk PRIMARY KEY (account_id);
ALTER TABLE relations ADD CONSTRAINT relations_pk PRIMARY KEY (follower_id, following_id);
I want to create a foreign key constraint from follower_id (relations) -> account_id (accounts), and the same with following_id.
ALTER TABLE relations ADD CONSTRAINT follower_id_fk FOREIGN KEY (follower_id) REFERENCES accounts (account_id) ON DELETE CASCADE
This foreign key is not accepted by the database. I get the following error:
ERROR: insert or update on table "relations" violates foreign key constraint "follower_id_fk"
DETAIL: Key (follower_id)=(4) is not present in table "accounts".
I understand this, because it's a composite primary key.
What I want to achieve:
When an account is deleted, I want to delete all the records where the account_id is the follower_id (ON DELETE CASCADE) AND where it is the following_id.
I could do this in my nodejs code or with a trigger function, but I don't know what will be the best performance-wise. Does anyone knows a/the best solution?
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withdrawn boolean not null,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.
Our database currently doesn't define primary keys on any tables. All of the id columns are simply unique indexes. I'm dropping those indexes and replacing them with proper primary keys.
My problem: In Postgres 8.4.7, one table in particular changes the data type from bigint to integer when I add the primary key to the table.
I've got the following table definition:
psql=# \d events
Table "public.events"
Column | Type | Modifiers
-----------------------+--------------------------+-----------------------------------------------------
id | bigint | not null default nextval('events_id_seq'::regclass)
[more columns omitted]
Indexes:
"events_id_unique_pk" UNIQUE, btree (id)
Foreign-key constraints:
"events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
"events_event_configs_id_fk" FOREIGN KEY (event_config_id) REFERENCES event_configs(id)
"events_pdu_circuitbreaker_id_fk" FOREIGN KEY (pdu_circuitbreaker_id) REFERENCES pdu_circuitbreaker(id)
"events_pdu_id_fk" FOREIGN KEY (pdu_id) REFERENCES pdus(id) ON DELETE CASCADE
"events_pdu_outlet_id_fk" FOREIGN KEY (pdu_outlet_id) REFERENCES pdu_outlet(id)
"events_sensor_id_fk" FOREIGN KEY (sensor_id) REFERENCES sensors(id)
"events_user_id_fk" FOREIGN KEY (clearing_user_id) REFERENCES users(id)
Referenced by:
TABLE "events" CONSTRAINT "events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
TABLE "event_params" CONSTRAINT "events_params_event_id_fk" FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
Triggers:
event_validate BEFORE INSERT OR UPDATE ON events FOR EACH ROW EXECUTE PROCEDURE event_validate()
This is what happens:
psql=# ALTER TABLE events ADD PRIMARY KEY (id);
NOTICE: ALTER TABLE / ADD PRIMARY KEY will create implicit index "events_pkey" for table "events"
ALTER TABLE
psql=# \d events
Table "public.events"
Column | Type | Modifiers
-----------------------+--------------------------+-----------------------------------------------------
id | integer | not null default nextval('events_id_seq'::regclass)
[more columns omitted]
Indexes:
"events_pkey" PRIMARY KEY, btree (id)
"events_id_unique_pk" UNIQUE, btree (id)
Foreign-key constraints:
"events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
"events_event_configs_id_fk" FOREIGN KEY (event_config_id) REFERENCES event_configs(id)
"events_pdu_circuitbreaker_id_fk" FOREIGN KEY (pdu_circuitbreaker_id) REFERENCES pdu_circuitbreaker(id)
"events_pdu_id_fk" FOREIGN KEY (pdu_id) REFERENCES pdus(id) ON DELETE CASCADE
"events_pdu_outlet_id_fk" FOREIGN KEY (pdu_outlet_id) REFERENCES pdu_outlet(id)
"events_sensor_id_fk" FOREIGN KEY (sensor_id) REFERENCES sensors(id)
"events_user_id_fk" FOREIGN KEY (clearing_user_id) REFERENCES users(id)
Referenced by:
TABLE "events" CONSTRAINT "events_clearing_event_ref_fk" FOREIGN KEY (clearing_event_id) REFERENCES events(id)
TABLE "event_params" CONSTRAINT "events_params_event_id_fk" FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
Triggers:
event_validate BEFORE INSERT OR UPDATE ON events FOR EACH ROW EXECUTE PROCEDURE event_validate()
I considered a few workarounds, but I'd really rather know why it's happening. There are a few other tables that also use bigint, so I don't want to just hack a solution in place.
This is scripted with Liquibase, but it happens directly in the Postgres console too.
Update
Two other points:
I can create a simple table with a bigint id and a unique index on id, add the primary key, and the column type stays the same.
All tables are empty at the time execution.
Could it have something to do with the constraints?
That's pretty interesting. I can't reproduce it with version 9.1.0 (yes, I should upgrade too!). But then I don't know precisely how the original table and sequence were created.
This page seems to allude to a similar automatic change of types between SERIAL and INTEGER: http://grover.open2space.com/content/migrate-data-postgresql-and-maintain-existing-primary-key
Could it be something like creating the table using SERIAL instead of BIGSERIAL, and then forcing the type to BIGINT? Something in between the sequence and primary key manipulations might have reset it.
I wasn't able to reproduce this the next day, even after reproducing it multiple times with witnesses the first time it occurred. I'm chalking it up to gremlins.
Can I drop primary key index without dropping primary key constraint in postgresql?
Your question is a bit confusing. I think you must mean this:
Can I drop an index on a column but still keep the uniqueness constraint on that column?
No. A uniqueness constraint requires an index. You can make your constraint into an ordinary non-primary index, but you can't make it not an index.
Also, read about primary keys in the documentation:
Technically, a primary key constraint is simply a combination of a unique constraint and a not-null constraint.
So if a column is a primary key it has by definition a unique constraint and therefore also an index. You cannot have a primary key that isn't an index.