How to create a primary key using the hash method in postgresql
hash, postgresql
Solution
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.
Problem
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 ```