On Postgres, a unique index is automatically created for primary key columns. From the docs,
When an index is declared unique, multiple table rows with equal
indexed values are not allowed. Null values are not considered equal.
A multicolumn unique index will only reject cases where all indexed
columns are equal in multiple rows.
From my understanding, it seems like this index only checks uniqueness and isn't actually present for faster access when querying by primary key id's. Does this mean that this index structure doesn't consist of a sorted table (or a tree) for the primary key column? Is this correct?
In theory a unique or primary key constraint could be enforced without the presence of an index, but it would be a painful process. The index is mainly there for performance purposes.
However some databases (eg Oracle) allow a unique or primary key constraint to be supported by a non-unique index. Primarily this allows the enforcement of the constraint to be deferred until the end of a transaction, so lack of uniqueness can be permitted temporarily during a transaction, but also allows indexes to be built in parallel and with the constraint then defined as a secondary step.
Also, I'm not sure how the internals work on a PostgreSQL btree index, but all Oracle btree's are internally declared to be unique either:
on the key column(s), for an index that is intended to be UNIQUE, or
on the key column(s) plus the indexed row's ROWID, for a non-unique index.
Quite the contrary, The index is created in order to allow faster access - mainly to check for duplicates when a new record is inserted but can also be used by other queries against PK columns. The best structure for uk indexes is a btree because during the insert the index is created - If the rdbms detects collision in the leaf he will raise a unique constraint violation.
Related
postgres 14
I have some table:
CREATE TABLE sometable (
id integer NOT NULL PRIMARY KEY UNIQUE ,
a integer NOT NULL DEFAULT 1,
b varchar(32) UNIQUE)
PARTITION BY RANGE (id);
But when i try to execute it, i get
ERROR: unique constraint on partitioned table must include all partitioning columns
If i execute same table definition without PARTITION BY RANGE (id) and check indexes, i get:
tablename indexname indexdef
sometable, sometable_b_key, CREATE UNIQUE INDEX sometable_b_key ON public.sometable USING btree (b)
sometable, sometable_pkey, CREATE UNIQUE INDEX sometable_pkey ON public.sometable USING btree (id)
So... unique constraints exist
whats the problem? how can i fix it?
On partitioned tables, all primary keys, unique constraints and unique indexes must contain the partition expression. That is because indexes on partitioned tables are implemented by individual indexes on each partition, and there is no way to enforce uniqueness across different indexes.
If you want to use partitioning, you have to sacrifice some consistency guarantees. There is no way around that. What you can do is create unique constraints on the partitions. That will guarantee uniqueness within each partition, but not global uniqueness.
This limitation is also mentioned in the docs
5.11.2.3. Limitations The following limitations apply to partitioned tables:
Unique constraints (and hence primary keys) on partitioned tables must
include all the partition key columns. This limitation exists because
the individual indexes making up the constraint can only directly
enforce uniqueness within their own partitions; therefore, the
partition structure itself must guarantee that there are not
duplicates in different partitions.
There is no way to create an exclusion constraint spanning the whole
partitioned table. It is only possible to put such a constraint on
each leaf partition individually. Again, this limitation stems from
not being able to enforce cross-partition restrictions.
https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE
Firstly, I have a table in database USERS with almost 30 Million records in it. I have different indices for each column. But some of the column have only 2 to 3 non null values while others are Null but still their index size is 847 MB a little less than the one index that contain unique value for each row.
Can anyone know why is it like this?
Secondly, in PostgreSQL we have a index for primary key index for each column by default what if we delete that index what will be the consequences?
What that index is really use for?
As i'm searching based on values in other columns only will it be safe to delete index for primary key?
NULL values are stored in indexes just like all other values, so the first part is not surprising.
You cannot delete the primary key index, what you could do is drop the primary key constraint. But then you cannot be certain that no duplicate rows get added to the table. If you think that is no problem, look at the many questions asking for help with exactly that problem.
Every table should have a primary key.
But it might be a good idea to get rid of some other indexes if you don't need them.
There is nothing called primary key index, seems to be you are talking about unique index.
First of all you need to understand the difference between primary key and index. You can have only one primary key in a table. Primary key would be your unique identifier of each column and does not allow nulls. Index is used to speed up your fetching process on particular column and you can have one null if it is unique index. Deleting unique index in your table will not impact any thing apart from performance. Its your way of design to have index or not
I was reading up on the BRIN index within PostgreSQL, and it seems to be beneficial to many of the tables we use.
That said, it applies nicely to a column which is already the primary key, in which case adding a separate index would negate part of the benefit of the index, which is space savings.
A PK is implicitly indexed, is it not? On that note, can it be done using a BRIN instead of a Btree, assuming the Btree is also implicit?
I tried this, and as expected it did not work:
create table foo (
id integer,
constraint foo_pk primary key using BRIN (id)
)
So, two questions:
Can a BRIN index be used on a PK?
If not, will the planner pick the more appropriate of the two if I have both a PK and a separate BRIN index (if performance means more to me than space)
And it's course possible that my understanding of this is incomplete, in which case I would appreciate any enlightenment.
Primary keys are a logical combination of NOT NULL and UNIQUE, therefore only an index type that supports uniqueness can be used.
From the PostgreSQL documentation (currently version 13):
Only B-tree currently supports unique indexes.
I'm not so sure BRIN would be faster than B-tree. It's a lot more space-efficient, but the fact that it's lossy and requires a secondary verification pass erodes any potential speed advantages. Once you are locked into having your B-tree primary key index, there's not much point to making a secondary overlapping BRIN index.
Is there any way to create a primary key using the hash method? Neither of the following statements work:
oid char(30) primary key using hash
primary key(oid) using hash
I assume, you meant to use the hash index method / type.
Primary keys are constraints. Some constraints can create index(es) in order to work properly (but this fact should not be relied upon). F.ex. a UNIQUE constraint will create a unique index. Note, that only B-tree currently supports unique indexes. The PRIMARY KEY constraint is a combination of the UNIQUE and the NOT NULL constraints, so (currently) it only supports B-tree.
You can set up a hash index too, if you want (besides the PRIMARY KEY constraint) -- but you cannot make that unique.
CREATE INDEX name ON table USING hash (column);
But, if you are willing to do this, you should be aware that there is some limitation on the hash indexes (up until PostgreSQL 10):
Hash index operations are not presently WAL-logged, so hash indexes might need to be rebuilt with REINDEX after a database crash if there were unwritten changes. Also, changes to hash indexes are not replicated over streaming or file-based replication after the initial base backup, so they give wrong answers to queries that subsequently use them. For these reasons, hash index use is presently discouraged.
Also:
Currently, only the B-tree, GiST and GIN index methods support multicolumn indexes.
Note: Unfortunately, oid is not the best name for a column in PostgreSQL, because it can also be a name for a system column and type.
Note 2: The char(n) type is also discouraged. You can use varchar or text instead, with a CHECK constraint -- or (if the id is so uuid-like) the uuid type itself.
This is kind of a general DB design question. If one has an associative entity table, i.e. a cross-reference, containing records that basically just consist of two FK references, should it be indexed in some way? Is it necessary to explicitly index that table, since the PKs in the associated tables are already indexed by definition? If one should index it, should it be a combination index, consisting of the two FK fields together?
Indexes on the referenced pk columns in the other tables do not cover it.
By defining the two fk columns as composite primary key of the "associative entity" table (as you should in most cases - provided that associations are unique), you implicitly create a multi-column index.
That covers all queries involving both or the first columns optimally.
It also covers queries on the second column, but in a less effective way.
If you have important queries involving just the second column, create an additional index on that one, too.
Read all the details about the topic at this related question on dba.SE.
Or this question on SO, also covering this topic.
Suppose your associative table has a schema such as:
CREATE TABLE Association
(
ReferenceA INTEGER NOT NULL REFERENCES TableA CONSTRAINT FK1_Association,
ReferenceB INTEGER NOT NULL REFERENCES TableB CONSTRAINT FK2_Association,
PRIMARY KEY(ReferenceA, ReferenceB) CONSTRAINT PK_Association
);
The chances are that your DBMS will automatically create some indexes.
Some DBMS will create an index for each of the two foreign keys and also a unique index for the primary key. This is slightly wasteful since the PK index could be used for accessing ReferenceA too.
Ideally, there will be just two indexes: the PK (unique) index and the (duplicates allowed) FK index for ReferenceB, assuming that the PK index has ReferenceA as the first column.
If a DBMS does not automatically create indexes to enforce the referential integrity constraints, you'll want to create the RI or FK duplicates-allowed index. If it doesn't automatically create an index to enforce the PK constraint, you'll want to create that unique index too. The upside is that you'll only create the indexes for the ideal case.
Depending on your DBMS, you might find it more effective to create the table without the constraints, then to add the indexes, and then to add the constraints (which will then use the indexes you created). Things like fragmentation schemes can also factor into this; I ignored them above.
The concept remains simple — you want two indexes in total, one to enforce uniqueness on both columns and provide fast access on the leading column, and a non-unique or duplicates-allowed index on the trailing column.