I have a table with a unique constraint on two fields, I also use this as an index for faster performance. I want to query a third field as part of this index but I don't want the third field to be part of the unique constraint. i.e. I don't want a new composite index just for the third field as it's quite large.
Is there a way to do this in Postgres? I presently create the unique constraint and get the index created for free, can I specify the three-field composite index and tell the unique constraint to use this index, and Postgres will figure out it can use this index as a UC?
You can use the INCLUDE option:
create unique index on the_table (column_1, column_2)
include (column_3);
Related
I have a question about a fundamental aspect of PostgreSQL.
Suppose I have two tables along the lines of the following:
create table source_data_property (
source_data_property_id integer primary key generated by default as identity,
property_name text not null
);
create table source_data_value (
source_data_value_id integer primary key generated by default as identity,
source_data_property_id integer not null references source_data_property,
data_value numeric not null
);
Suppose I write a very simple query that just performs a basic join:
select
sdp.source_data_property_id,
sdp.property_name,
sdv.source_data_value_id,
sdv.data_value
from source_data_property as sdp
join source_data_value as sdv using (source_data_property_id)
;
For optimal query performance, is it necessary to add an index on the source_data_property_id column in the source_data_value table? My original thought was no, because the source_data_property_id is already indexed in the source_data_property table, but after thinking about it a bit I'm not so sure.
For optimal query performance, is it necessary to add an index on the source_data_property_id column in the source_data_value table?
In general yes, make indexes for your foreign keys. However...
A very small table won't get any advantage from indexes and Postgres will do a seq scan instead.
Similarly it depends on what sort of queries you're doing. In your example you're fetching every row in source_data_property which will also fetch every row in source_data_value. Using an index is slower and Postgres will do a seq scan instead.
I have a table ideas with columns idea_id, element_id and element_value.
Initially, I had created a composite primary key(ideas_pkey) using all three columns but I started facing size limit issues with the index associated with the primary key as the element_value column had a huge value.
Hence, I created another unique index hashing the column with possible large values
CREATE UNIQUE INDEX ideas_pindex ON public.ideas USING btree (idea_id, element_id, md5(element_value))
Now I deleted the initial primary key ideas_pkey and wanted to recreate it using this newly created index like so
alter table ideas add constraint ideas_pkey PRIMARY KEY ("idea_id", "element_id", "element_value") USING INDEX ideas_pindex;
But this fails with the following error
ERROR: syntax error at or near "ideas_pindex"
LINE 2: ...a_id", "element_id", "element_value") USING INDEX ideas_...
^
SQL state: 42601
Character: 209
What am I doing wrong?
A primary key index can't be a functional index. You can instead just have a unique index on your table, or create another column storing the md5() of your larger column and use it in the PK.
That being said, there is also another error in your query: If you want to specify an index name, you can't specify the PK columns (they are derived from the underlying index). And if you want to specify the pk columns, you can't specify the index name/definition, as it will be automatically created. See the doc
I have a table "locations" with "from" and "to" columns with "point" type.
I'm only able to use "gist" indexes on those columns as the b-tree is not available for "point" type.
I would like to have a unique index on both of the columns (to ensure there is no same location stored).
This is not possible due to error "access method "gist" does not support unique indexes".
Is it somehow possible to achieve this? I could workaround it by creating regular text column storing "from_lat,from_lng:to_lat,to_lng" and add unique index on it, but is there a better way?
You can use an exclusion constraint. A unique constraint (or index) is essentially just a special case of an exclusion constraint.
An exclusion constraints an be defined using GIST:
alter table locations
add constraint unique_points
exclude using gist ("from" with ~=, "to" with ~=);
The operator ~= checks for the equality of two points
You may use the "exclude" constraint, but this is not a unique index nor a primary key at all. Furthermore an upsert is impossible, because to resolve "on conflict (...) do update" a primary or unique key is required.
Referring https://www.postgresql.org/docs/13/indexes-unique.html it is impossible to use any range type as unique key (or pkey, including violations by overlapping ranges etc.)
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.
I'm new to postgres. I wonder, what is a PostgreSQL way to set a constraint for a couple of unique values (so that each pair would be unique). Should I create an INDEX for bar and baz fields?
CREATE UNIQUE INDEX foo ON table_name(bar, baz);
If not, what is a right way to do that? Thanks in advance.
If each field needs to be unique unto itself, then create unique indexes on each field. If they need to be unique in combination only, then create a single unique index across both fields.
Don't forget to set each field NOT NULL if it should be. NULLs are never unique, so something like this can happen:
create table test (a int, b int);
create unique index test_a_b_unq on test (a,b);
insert into test values (NULL,1);
insert into test values (NULL,1);
and get no error. Because the two NULLs are not unique.
You can do what you are already thinking of: create a unique constraint on both fields. This way, a unique index will be created behind the scenes, and you will get the behavior you need. Plus, that information can be picked up by information_schema to do some metadata inferring if necessary on the fact that both need to be unique. I would recommend this option. You can also use triggers for this, but a unique constraint is way better for this specific requirement.