NULL is always different from another NULL. If you're inserting a NULL value into a table and you have a unique constraint, the NULL value is considered to be distinct on its own. In Postgres 14 and older versions unique constraints always treat NULL values as not equal to other NULL values. Now the "val1" that's a NOT NULL column, you couldn't actually insert NULL values into that column, but for the "val2" column the difference here is that in this second table, if you insert a NULL value into this nullable column, then it will be treated differently. What that does is it treats NULL values in the unique constraint differently. This is the exact same schema, except the unique constraint here uses this new NULLS NOT DISTINCT value. The second table that Ryan creates here is a "null_new_style" table. If you are on a modern Postgres version, this is actually the preferred style to create an auto incrementing value in Postgres. Separately, you'll also note here that for the "id" column, that's not related to the constraint, he's using the newer syntax here that is GENERATED BY DEFAULT AS IDENTITY, instead of using a sequence in Postgres. There's one table which he called "null_old_style", that's just using a standard unique constraint on two columns. For this, he has two different tables that he's looking at. Ryan goes into more details of what that means. 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. ![]() If you don't know, Postgres 15 is currently in beta season, beta two was recently released. UNIQUE constraints and NULL valuesįirst of all, in this post by Ryan Lambert on the RustProof Labs blog, he talks about the new feature for unique constraints. What we have discussed in this episode of 5mins of Postgres The upcoming SQL Standard and NULLS NOT DISTINCTīetter performance with Parallel SELECT DISTINCT.Rejecting duplicate NULL values in Postgres 15 with NULLS NOT DISTINCT.The behavior in older Postgres versions.When set to a positive value, ANALYZE will assume that the column contains exactly the specified number of distinct nonnull values. n_distinct affects the statistics for the table itself, while n_distinct_inherited affects the statistics gathered for the table plus its inheritance children. Currently, the only defined per-attribute options are n_distinct and n_distinct_inherited, which override the number-of-distinct-values estimates made by subsequent ANALYZE operations. ![]() This form sets or resets per-attribute options. SET STATISTICS acquires a SHARE UPDATE EXCLUSIVE lock. For more information on the use of statistics by the PostgreSQL query planner, refer to Section 14.2. The target can be set in the range 0 to 10000 alternatively, set it to -1 to revert to using the system default statistics target ( default_statistics_target). ![]() This form sets the per-column statistics-gathering target for subsequent ANALYZE operations. sequence_option is an option supported by ALTER SEQUENCE such as INCREMENT BY. These forms alter the sequence that underlies an existing identity column. If DROP IDENTITY IF EXISTS is specified and the column is not an identity column, no error is thrown. Like SET DEFAULT, these forms only affect the behavior of subsequent INSERT and UPDATE commands they do not cause rows already in the table to change. These forms change whether a column is an identity column or change the generation attribute of an existing identity column. RENAME CONSTRAINT constraint_name TO new_constraint_nameĪLTER TABLE ALL IN TABLESPACE name ]ĪTTACH PARTITION partition_name AS IDENTITY
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