Postgres unique index vs constraint. 以下は、双方確認できる。.

But not the other three unique constraints / indexes. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. Defining a UNIQUE Constraint in PostgreSQL. These additional indexes are not only unnecessary, but actually counterproductive. It happens to be implemented via the creation. create unique index Table1_Field1Field2. Partial indexes are a specialized feature, but there are several situations in which In PostgreSQL 14, UNIQUE constraints are the second most popular behind only PRIMARY KEY, used in 21. All the columns of the index will be included in the constraint. The key field (s) for the index are specified as column names, or alternatively Jan 14, 2021 · I created a table in postgres that includes various columns that based on an import from a csv. . (See CREATE INDEX for more information. If you look at the model without any. create or replace function array_sort (arr anyarray) returns anyarray immutable as. x: CREATE UNIQUE INDEX constraint_name ON table_name (columns); 11. One option with Postgres 9. > Unique indexes can be partial, i. See: How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use? edited Apr 6, 2021 at 0:01. Partial Indexes #. PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a Dec 30, 2021 · 3. Marvellous. I want to allow NULL in MenuId to store a favorite that has no associated menu, but I only want at most Nov 24, 2020 · And, unfortunately, Postgres does not have an "array sort" function. PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a . In this case SET CONSTRAINTS will act on all matches. Of course, this is easy enough to do in SQL using array_agg() and unnest(). But it doesn't work the other way around, a unique index does not create a constraint. Allowed. conditional) UNIQUE constraint - however, you can create a partial unique index. From the Postgres 14 docs: When an index is declared unique, multiple table rows with equal indexed values are not allowed. So, col1 and col2 can repeat, but not at the same time. An exclusion constraints an be defined using GIST: The operator ~= checks for the equality of two points. 1) Unique indexes for a primary key column and a column with a unique constraint. In PostgreSQL, a CHECK constraint ensures that values in a column or a group of columns meet a specific condition. constraint is implemented by creating a unique index, and so there is. Finally, unique constraints are SQL-standard. Erwin Brandstetter. With the syntax for creating an index, you may have better control defining a unique index to specify clustered/nonclustered, included columns, filegroup, index filtering (SqlSvr2008), etc. – joop. Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. OR. Convert hex in text representation to decimal number; Would index lookup be noticeably faster with char vs varchar when all values are 36 chars; Before Postgres 10 the use of hash Dec 27, 2011 · The docs advocate this method, Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. Create array_sort helper function (it might be useful for other cases too) and an unique index using it. When an index is declared unique, multiple table rows with equal indexed values are not allowed. In PG a unique. FROM information_schema. HasAlternateKey(x => x. Jan 17, 2022 · Postgres unique constraint vs index. SQL Postgres 唯一约束 vs 索引. The Many Mysteries of Merge Description. FOREIGN KEY directive is used to create the constraint in an “inline” fashion within the CREATE TABLE definition. HasName("UXC_IDTypes_Code"); Replace it with this code: Jun 24, 2022 · A former engineer working on the project had such a constraint: ALTER TABLE products ADD CONSTRAINT unique_active_name EXCLUDE (name WITH =) WHERE (status = 'active' AND name <> ''); I am wondering if it is equivalent to such a partial unique index: Dec 27, 2010 · A UNIQUE INDEX is both a unique constraint and an index that can be used like any other index. > unique constraints cannot. I want to create a unique constraint similar to this: ALTER TABLE Favorites. May 22, 2023 · While this blog focuses on unique indexes/constraints, most of the content applies to other indexes and constraints. really not any interesting difference. UNIQUE (email); Note that if there are existing duplicates in the column, PostgreSQL won’t let you add the constraint until those duplicates are resolved. See Concurrency Control in the PostgreSQL manual. add constraint Table1_UC1. This means we're already getting a unique index with each unique constraint, and adding another unique index is simply duplicating the one underneath our unique constraint. A check constraint allows you to enforce data integrity rules at the database level. All this is also true of a PRIMARY KEY, which is also implemented using a UNIQUE index. The unique constraint is implemented using an index, so checking for duplication is O (log n) not O (n). Now, right-click on the table where you want to add the unique constraints and click on 'Properties' in the menu. extra; LINE 2: on conflict (kv_key_value) do update set extra=exclude When an index is declared unique, multiple table rows with equal indexed values are not allowed. 9% of schemas. It is the indisclustered attribute in pg_index catalogue. Sep 29, 2023 · The UNIQUE constraint is similar to the primary key except that the UNIQUE constraint accepts the null values. The key to success: NULL handling. 3 doesn't support expressions in UNIQUE constraints so an index, that supports expressions, has to be used: create unique index unique_data_url_index on mytable (md5(data_url)); Aug 17, 2010 · See last paragraph of UNIQUE constraints in the PostgreSQL documentation: In general, a unique constraint is violated when there are two or more rows in the table where the values of all of the columns included in the constraint are equal. The name of the table to modify. This is effectively pretty much the same as a unique constraint, because such constraints are implemented Jul 14, 2022 · In this post here, he details the change that Peter Eisentraut made in Postgres 15 to allow unique constraints and indexes to treat NULL values as not distinct. When Pg creates an implicit index it will emit a NOTICE -level message that you can see in psql and/or the system logs, so you can see when it happens. It does so by checking an index, not the individual rows. However, two null values are not considered equal in this comparison. : ALTER TABLE sample_table. user330315. ADD CONSTRAINT unique_email. Dec 19, 2023 · If you are using ON CONFLICT (id), you can still get a unique constraint violation on a different constraint. said index is not part of the model. About the deadlocks, please add the details from the log to the question. A PRIMARY KEY constraint automatically has a UNIQUE constraint. This is the table that you wish to add a unique constraint to. PostgreSQL doesn't define a partial (i. > be included in a query so that PostgreSQL knows to use that index) whereas. Specifying NULLS NOT DISTINCT on unique indexes / constraints will cause NULL to be treated as not distinct, or in other words, equivalently. as far as I know, the options for enforcing partial constraints in postgres are unique index or exclude constraint. 以下は、双方確認できる。. Sep 19, 2020 · Unique constraints will create a unique index in the background anyway, it’s the only way to enforce the constraint in a performant manner. A unique constraint implies the creation of a unique index, but not vice versa. They strike a balance between data integrity and flexibility. May 19, 2015 · Instead of using the text data type, we can use the citext (case insensitive text) type! First we need to enable the citext extension: Then we'll need to change the email data type in the users table: Now our existing UNIQUE constraint should be case insensitive! DETAIL: Key ( email)=(TEST@example. The unique index is an implementation artifact that we use to implement the constraints. pg_indexes. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. select array_agg(x order by x) from unnest(arr) x; Jan 1, 2023 · Simply right click any table to View Structure, to view a list of all indexes and constraints. 📚 This blog is part of a series about PostgreSQL migration best practices May 27, 2016 · A UNIQUE constraint in Postgres creates an index - for the easiest enforcement of the constraint. You may use the "exclude" constraint, but this is not a unique index nor a primary key at all. Nov 2, 2012 · VACUUM work by autovacuum cleans the dead index entries up later. Oct 3, 2018 · UNIQUE(col2, col1) -- reversed column ordering. (At present, only b-tree supports it. The trigger solution is not transparent as it is actually modifying the data. By the logic employed, an index is also a key but only when unique. create the column first, generate unique data for it, then add NOT NULL afterwards. Consider this contrived scenario of one table with one column that has a unique constraint (and index): Dec 4, 2020 · While Postgres doesn't allow a partially unique constraint, it does support a partial unique index: create unique index unique_row on myTable(content_id, brand_id) where not is_archived; See Partial Indexes in the Postgres documentation. PostgreSQL has chosen to allow a user to create a unique index directly, instead of only via a constraint, but one should not do so. The unique constraint is a type of object that's defined by the SQL standard. But what about on a distributed system where there are multiple computers storing many many rows of In some databases (I'm uncertain about postgres specifically) unique constraints give more information to the optimizer than a simple unique index, and allow for more query transformations (e. When the constraint is not unique you need to use a regular or non unique index. ADD CONSTRAINT constraint_name UNIQUE (column1, column2, column_n); table_name. Use a unique constraint unless you have a good reason to create the unique index directly. Question: Can anyone prove a unique index could be better than a non unique index from an execution plan and show us the queries and executions plan? To my knowledge from unique index of sql-server not only be a constraint but also can be better performance than non unique index. Dec 26, 2017 · How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use? md5() would be a simple option as hash function. Constraint is also another type of index. Let’s explore some examples of using the PostgreSQL unique indexes. ADD CONSTRAINT my_unique_constraint UNIQUE(my_column) Jun 18, 2024 · The CONSTRAINT. defined with a where clause (that must. Mar 4, 2020 · 1. CREATE INDEX constructs an index on the specified column (s) of the specified relation, which can be a table or a materialized view. An access method that supports this feature sets amcanunique true. table_constraints a1 where a1. answered Apr 15, 2015 at 16:17. A partial index is an index built over a subset of a table; the subset is defined by a conditional expression (called the predicate of the partial index). Jan 25, 2011 · 37. A constraint is preferable in most cases because it When you define a primary key or a unique constraint for a table, PostgreSQL automatically creates a corresponding unique index. Furthermore an upsert is impossible, because to Unique. Other than that, they're supposed to be pretty much the same. And the only difference between a unique constraint and a unique index is INCLUDE columns. The index covers the columns that make PostgreSQL allows you to create a UNIQUE constraint to a group of columns using the following syntax: c1 data_type, c2 data_type, c3 data_type, UNIQUE (c2, c3) The combination of values in the columns c2 and c3 will be unique across the whole table. PostgreSQL automatically creates an index for each unique constraint and primary key constraint to enforce uniqueness. So then we'd start with IK_columnname for unique indexes and IX_columnname for non-unique indexes. 4 would be to create a unique index on the MD5 hash of the JSON value. PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. But if you have non-unique entries in your table, it will fail. The only way you can prevent that is to either declare the column as NOT NULL in order to force a value for it, or reduce the unique columns to (user_id, class_type_id) answered May 3, 2014 at 20:10. So by Django's contract, unique=True implies db_index=True, and db_index=True means that Django has to create the text_pattern_ops index to support To make a column unique in a PostgreSQL table, you can use the ALTER TABLE statement with the ADD CONSTRAINT clause. The documentation for unique constraint specifies that a unique index is created under the hood upon creation of a unique constraint. If those two reasons to create indices happen to overlap, great! May 3, 2014 · This happens because of the NULL value in the created_at column. This implies there can be data in the table but not in the index and thus. PostgreSQL UNIQUE index examples. In PostgreSQL the clustered attribute is held in the metadata of the corresponding index, rather than the relation itself. May 5, 2016 · There are no differences between all the SQL statements, except the third one you're using a different name for the unique index, that's all. Introduction to PostgreSQL CHECK constraints. Postgres 14 comes with a built-in hash function for records (including anonymous records!), which is substantially cheaper than my custom function below. PostgreSQL uses unique indexes to implement unique constraints, so the effect is the same, with an important caveat: you can't perform upserts ( ON CONFLICT DO UPDATE) against a unique index like you would against a unique See full list on medium. PostgreSQL enforces SQL uniqueness constraints using unique indexes, which are indexes that disallow multiple entries with identical keys. Because PostgreSQL does not require constraint names to be unique within a schema (but only per-table), it is possible that there is more than one match for a specified constraint name. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. Index Uniqueness Checks #. com) already exists. The index contains entries only for those table rows that satisfy the predicate. com Jul 22, 2014 · The solution is to remove your data, add the column then add the data back. So I would not need to generate a separate index to speed up queries like this in either case. Open Table Properties. Feature Description UNIQUE NULLS NOT DISTINCT. e. Jul 21, 2010 · When you create a unique constraint, SQL Server will automatically create a unique index for you. Date: 13 October 2003, 13:40:16. the optimizer might be able to see a unique constraint on a correlated field used to match in an EXISTS clause, and convert, say, a "nested loop semi Jan 4, 2024 · The following command shows how to add a unique constraint to the email column of the users table: ALTER TABLE users. Otherwise it's an index. Indexes are primarily used to enhance database performance (though inappropriate use can result in slower performance). ); Looking at the catalogue we see the constraint name: SELECT conname FROM pg_constraint WHERE conname LIKE 'test%'; -- "test_pkey". UNIQUE constraints are implemented using a unique B-tree index in Postgres. Note: The preferred way to add a unique constraint to a table is Feb 28, 2007 · the notion of a unique constraint is outside its bounds. Having the name of the index, we can reindex it: REINDEX INDEX test_pkey; As suggested by @Josh Kupershmidt and @JoeNahmias the solution is to use UNIQUE on md5 hash of the long value. Also, you can create things like partial unique indexes, but you cannot do that on a constraint. Id BIGSERIAL PRIMARY KEY. Jan 6, 2015 · 26. Basically, create unique indices to enforce uniqueness rules. constraint_type in ('PRIMARY KEY' , 'UNIQUE') SQLを実行してもらえればわかるが、インデックスのみの場合、カラムの指定 ADD table_constraint_using_index # This form adds a new PRIMARY KEY or UNIQUE constraint to a table based on an existing unique index. I would like to set up a table in PostgreSQL such that two columns together must be unique. $$. Add Unique Constraint. The heart of the issue is this guarantee made in the Django documentation: Note that when unique is True you don’t need to specify db_index, because unique implies the creation of an index. When a unique constraint is created a Mar 25, 2020 · Yes and no. Every index, including the indexes used by PRIMARY KEY and UNIQUE constraints, incurs a penalty on write performance. ADD CONSTRAINT Favorites_UniqueFavorite UNIQUE(UserId, MenuId, RecipeId); However, this will allow multiple rows with the same (UserId, RecipeId), if MenuId IS NULL. What makes a primary key different from a unique index is the way NULL entries are handled. The syntax for creating a unique constraint using an ALTER TABLE statement in PostgreSQL is: ALTER TABLE table_name. Oct 4, 2013 · Uniqueness is a constraint. However PostgreSQL 9. PostgreSQL UNIQUE Constraint Syntax. According to PostgreSQL Documentation you can create unique partial index which will be effectively the same as creating unique partial constraint on your table: CREATE UNIQUE INDEX some_index ON some_table (col_a) WHERE (col_b is null); Using this technique you can create 2 separate unique indexes for admin and non-admin users. Jun 26, 2017 · Yes. *. In summary, a unique constraint in PostgreSQL is a way to ensure that the values in a particular column or set of columns are unique. drop_all() methods do this by default, using a topological sort of all the Table objects involved such that tables are created and dropped in order of their foreign key dependency (this sort is also available via the Apr 15, 2015 · 13. Index is used for faster access. ) Unique constraints and primary keys are not inherited in the current implementation. Dec 16, 2023 · primary key (id), foreign key (test_id) references test(id) ); my goal is to impose a partial unique index (constraint) on field cid from table details provided that field stat from table test is equal to ACTIVE. table_constraints. There's one table which he called "null_old_style", that's just using a Jul 9, 2018 · Whereas a CHECK constraint evaluates an expression based on a single row of the table, an EXCLUDE constraint evaluates a comparison of two rows in the table. SELECT id, col3 FROM foo WHERE col1 = 'stack' AND col2 Apr 1, 2021 · A unique constraint (or index) is essentially just a special case of an exclusion constraint. It will create only one index on two columns and both the column combinedly can not contain duplicate in the entire table. Also, it must be a b-tree index with default sort ordering. create unique default data for the new column. Just pick the name you prefer for the unique key and feel free to use any of your queries. Let's take a look at a simple example: Jan 27, 2019 · Let's create a table with a Primary Key constraint which is also an Index: CREATE TABLE test(. Postgres Unique Constraint Summary. Quote from the manual. g. indexes, because it'd be awful darn expensive to enforce the constraint. IMHO there is no need to enforce uniqueness by means of constraints at this level. In above statement, mytable is the name of the table you want to modify, column_name is the name of the column you want to make unique, and unique_column_name is a name you choose for the unique Feb 10, 2024 · All table_name unique indexes that, without regard to order, contain exactly the conflict_target-specified columns/expressions are inferred (chosen) as arbiter indexes. Here is the how to add a unique constraint to your table in PostgreSQL 9. Null values are not considered equal. Since the csv can be re-uploaded with some changes and I don't want the re-upload to create duplicates on rows that didn't change, I added a constraint and unique index with majority of the columns included in them. Jan 7, 2021 · 2. Jan 17, 2017 · Create an expression based UNIQUE INDEX: CREATE UNIQUE INDEX people_data_pos_idx ON peoples( (data->>'pos') ) ; If, at this point, you try to insert the following piece of data into your table (with an already existing ->>pos): Jun 13, 2016 · Before getting started, let me briefly describe SQL Server unique indexes vs. answered Jan 10, 2023 at 19:53. Here's an example SQL statement. select * from pg_indexes. Nov 13, 2023 · Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. For this delete this code in the configuration class: // Create the unique constraint builder. A UNIQUE constraint guarantees the uniqueness of data across one or more columns in a table. Jan 18, 2021 · This is obviously a convoluted solution. Aug 8, 2014 · The standard names for indexes in PostgreSQL are: {tablename}_{columnname(s)}_{suffix} where the suffix is one of the following: pkey for a Primary Key constraint; key for a Unique constraint; excl for an Exclusion constraint; idx for any other kind of index; fkey for a Foreign key; check for a Check constraint; Standard suffix for sequences is This is not true of a unique index. You can create the unique constraint in PostgreSQL in multiple ways. Moreover, you can have as many unique constraints as you want on a single table. A check constraint uses a boolean expression to evaluate the values, ensuring that only valid data Oct 5, 2013 · Steve Grey-2 wrote. But user-defined functions are. This is consistent with the SQL Standard handling of NULL in general, where NULL is In DML statements, enabled unique constraints on a logged table are checked at the end of a statement, but unique indexes are checked on a row-by-row basis, thereby preventing any insert or update of a row that might potentially violate the uniqueness of the specified column (or for a multiple-column column constraint or index, the column list). create_all() and MetaData. There can be multiple values of either value, so long as there are not two that share both. The index is created and maintained automatically, and is used for all purposes like a plain unique index by Postgres. A UNIQUE constraint is implemented with the help of a unique index - a b-tree index with default ascending sort ordering over all involved columns. I would imagine that other DBMSes also enforce uniqueness by means of. Also, create indices to reflect query patterns. You can view, edit, delete, and create constraints without knowing any SQL. Postgres checks that the product_no has not already been inserted. 5. Every time the user inserts a new row, PostgreSQL checks if the value already exists in the table if UNIQUE constraints are used. For example, you can create indexes for every year based on the date column Aug 2, 2016 · The logical key seems to be (schemaname, constraint_name): the names are supposed to be unique within a schema. Now, to add a unique constraints, click on the + button to add a row in a grid. We would like to show you a description here but the site won’t allow us. This is not reflected in pg_catalog. Note, however, that clustering relations within postgres is a one-time action: even if the attribute is true, updates to the table do not maintain the sorted PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. So, start with: create function array_sort(anyarray) returns anyarray as $$. property is a constraint and so a "unique index" without a corresponding. Ok. So it might happen that inserting data into such a table fails not because of the violation of the unique constraint, but because the JSON value is too big. You can add multiple duplicates for colA but considering that the colB is different for each row having same colA and vice-versa. The MetaData. Think of it like a generalised UNIQUE constraint: instead of "no two rows can be equal", you can say things like "no two rows overlap", or even "no two rows can be different". In your case, ON CONFLICT (id, core_id) determines (only!) the multicolumn unique constraint id_core_id_unique. ); From what I understand both commands will generate an index on the two columns to enforce the unique constraint but with different ordering. Then what is the difference in these two statements? alter table Table1. By default, NULL values are treated as distinct entries. For a non-schema-qualified name, once a match or matches have been found in some schema in the search path Sep 27, 2018 · Describing the above table, the output of "Indexes:" is slightly different ("UNIQUE CONSTRAINT" vs "UNIQUE" in previous example): However I am still unable to specify the constraint name as the conflict target: on conflict (kv_key_value) do update set extra=excluded. Automatically created indexes are visible in \d Nov 1, 2014 · What is the purpose of adding the CONSTRAINT if the UNIQUE INDEX already exists? I tried testing around a bit and it seems that creating the UNIQUE INDEX was sufficient to ensure uniqueness - trying to add duplicate data resulted in duplicate key value violates unique constraint 'package_tmp_id_idx'. 在本文中，我们将介绍SQL数据库中的唯一约束和索引在PostgreSQL中的使用场景和区别。唯一约束和索引是SQL中用于强制数据一致性和提高查询性能的重要工具。 阅读更多：SQL 教程. unique constraints: A unique index ensures that the values in the index key columns are unique. Here is an alternative. select array_agg(distinct n order by n) Jul 11, 2022 · In Postgres 14 and prior, unique constraints treated NULL values as not equal to other NULL values. The NULLS NOT DISTINCT option modifies this and causes the index to treat nulls as equal. The solution to this is to not use a unique constraint in entity framework but to use a unique index. Yes. for reference another answer with more info: Create unique constraint with null columns. Aug 28, 2020 · PostgreSQL provides the user with a UNIQUE constrain that is used to make sure that values stored in a column or a group of columns are unique across rows in a table. A unique index (or constraint) allows multiple rows with NULL in them. 以下、は制約のみ. For this, he has two different tables that he's looking at. Nulls are obviously tricky and can be interpreted in multiple ways (unknown value, absent value, etc. Thanks for the answer. All of them creates a unique key/index, constraint md5_constraint unique (query_md5) has the same effect. Yes, you can. Code) . Sep 5, 2021 · Create a UNIQUE index or constraint based on a cheap and sufficiently unique hash value of the row (reduced to defining columns). Jun 9, 2009 · PostgreSQL automatically creates indexes on primary keys and unique constraints, but not on the referencing side of foreign key relationships. of a unique index since an index is quickly able to search all existing. answered Mar 4, 2020 at 20:06. Unique Indexes can have where clauses. This is important because people often use additional indexes on primary keys or unique columns. SELECT a1. constraint_name. 8. Again click on the Jan 9, 2024 · If you already have a primary key on the partition, you can add a primary key on the partitioned table by first adding an invalid primary key on only the partitioned table, then attaching the primary key index on the partitions as partitions to that index. You can use unique indexes with additional features that you cannot create constraints for, such as partial indexes. First, create a table called Jun 20, 2022 · Constraints in PostgreSQL are validated immediately row-by-row by default, which might be confusing when updating multiple values in columns that have a uniqueness constraint. From the documentation: Note: The preferred way to add a unique constraint to a table is ALTER TABLE ADD CONSTRAINT. A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index. 唯一约束（Unique Constraint） 3. Oct 5, 2013 · PostgreSQL has chosen to allow a user to create a unique index directly, instead of only via a constraint, but one should not do so. The value of the column c2 or c3 needs not to be unique. values in order to determine if a given value already exists. The index cannot have expression columns nor be a partial index. Jan 31, 2017 · The bottom line is, PostgreSQL does what it does with nulls because the SQL standard says so. constraint is an improper model. Ryan goes into more details of what that means. unique (Field1, Field2) and. For instance: id int PRIMARY KEY AUTOINCREMENT, col1 int NOT NULL, col2 int NOT NULL. Jun 3, 2013 · If deferrable constraint is not needed, it is as simple as creating unique index with function, e. ) Columns listed in the INCLUDE clause are not considered when You could use a unique index instead and go for "IXU". pg_constraint, it only has a non-unique index on (conname, connamespace). : CREATE UNIQUE INDEX my_unique_index ON sample_table(UPPER(my_column)); Deferred constraint check requires creating the constraint explicitly, e. But those are not allowed in an index definition. Jun 21, 2016 · UNIQUE Constraint is preferred over UNIQUE Index. Thus, it is not necessary to create an index explicitly for primary key columns. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint. ), and so when the SQL standard was initially written, the authors had to make some calls at certain places. The uniqueness. 64. PostgreSQL Documentation - Unique Indexes. The use of indexes to enforce unique constraints could be considered an implementation detail that should not be accessed directly. In the popup, go to the 'Constraints' tab and click on the Unique tab, as shown below. The UNIQUE constraint ensures that all values in a column are different. sql. mg ho lk jz np ih lz qr nm fd