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In PostgreSQL 18, create a hash index with CREATE INDEX ... USING hash, inspect its definition and access method through psql or the system catalogs, and remove it with DROP INDEX. Hash indexes support equality comparisons only; they are single-column, cannot enforce uniqueness, and are not automatically faster than B-tree indexes.
Create a hash index
Specify USING hash in the index definition. If you omit the access method, PostgreSQL uses B-tree.
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CREATE INDEX users_email_hash_idx
ON public.users USING hash (email);The example creates an index on one column of public.users. Hash indexes support a single column and cannot be unique, so do not use CREATE UNIQUE INDEX ... USING hash to enforce uniqueness. Choose an index name that is clear and does not conflict with another relation in the table’s schema.
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Choose a regular or concurrent build
A regular CREATE INDEX blocks writes to the table until the build completes, while reads can continue. On a live table, a concurrent build permits ordinary inserts, updates, and deletes during construction:
CREATE INDEX CONCURRENTLY users_email_hash_idx
ON public.users USING hash (email);In PostgreSQL 18, concurrent creation requires two table scans, waits for relevant transactions, and takes longer. It cannot run inside a transaction block, and only one concurrent index build can run on a table at a time. A failed concurrent build can leave an invalid index: it is ignored for queries but can still add update overhead. Check the index status, then remove the invalid index and retry, or consider REINDEX INDEX CONCURRENTLY where appropriate.
A concurrent build on a partitioned table is not supported as one operation in PostgreSQL 18. The documented approach is to build indexes concurrently on individual partitions and attach them using the supported procedure.
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Inspect an index and confirm its method
Use psql
In psql, di lists indexes, while di+ adds details such as disk size. To see a table’s indexes and definitions, use:
d public.usersA failed concurrent build may be shown as INVALID in the table description.
Query the catalog
The pg_indexes view provides the schema, table name, index name, tablespace, and reconstructed index definition. To identify the access method directly, join the index relation in pg_class to pg_am through pg_class.relam:
SELECT ns.nspname AS index_schema,
idx.relname AS index_name,
am.amname AS index_method,
pg_get_indexdef(idx.oid) AS index_definition
FROM pg_class AS idx
JOIN pg_namespace AS ns ON ns.oid = idx.relnamespace
JOIN pg_am AS am ON am.oid = idx.relam
WHERE idx.relkind = 'i'
AND ns.nspname = 'public'
ORDER BY idx.relname;This returns ordinary index relations in the public schema and labels their method, such as hash or btree. Add a filter for the target table or index when needed. Partitioned index parents use a different relation kind, so include them separately if you are checking partitioned indexes.
Decide whether a hash index fits the workload
Hash indexes are candidates for equality predicates such as email = ..., not range predicates such as email > .... B-tree indexes support equality and ordered or range comparisons, can use multiple key columns, and can enforce uniqueness. Choose based on the operators and semantics your application needs, not on the index name.
PostgreSQL stores only a four-byte hash value in each hash-index entry, not the original indexed value. As a result, scans are lossy and must recheck matching table rows; hash indexes can also be used in bitmap index scans. PostgreSQL notes that hash indexes may be smaller than B-trees for longer values, including UUIDs and URLs, but that does not by itself establish a speed advantage. Bucket growth, overflow behavior, and many rows mapping to a bucket can also affect block accesses.
| Consideration | Hash | B-tree |
|---|---|---|
| Supported comparisons | Equality only | Equality and ordered/range comparisons |
| Key columns and uniqueness | One column; cannot be unique | Can use multiple key columns and can be unique |
| Index entries and scans | Stores a four-byte hash value; scans require table-row rechecks | Stores ordered keys |
Practical performance depends on value length, distinctness and collision distribution, table size, growth and update patterns, and query selectivity. Index existence alone does not show that the planner will use it or that it will help.
Check the plan and measure carefully
Refresh statistics when they are stale, then inspect a representative equality lookup:
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ANALYZE public.users;
EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM public.users WHERE email = '[email protected]';EXPLAIN shows the planner’s selected plan. EXPLAIN ANALYZE executes the query and reports actual rows and timing alongside estimates; it has measurement overhead and excludes client network transfer. Be especially cautious with statements that modify data or have side effects. Measure representative data and workload rather than extrapolating from a small test, and do not force planner settings as proof that an index is useful.
Drop a hash index
Remove an index by name:
DROP INDEX public.users_email_hash_idx;The index owner must run the command. By default, RESTRICT prevents removal when dependent objects exist. Use CASCADE only after reviewing what dependent objects it will remove. If an already-absent index should not cause an error, use DROP INDEX IF EXISTS; PostgreSQL reports a notice instead.
Drop concurrently on an active table
For a live table, concurrent removal avoids locking out concurrent selects, inserts, updates, and deletes while it waits for conflicting transactions:
DROP INDEX CONCURRENTLY public.users_email_hash_idx;A regular drop takes an ACCESS EXCLUSIVE lock on the table and can block other access until it completes. In PostgreSQL 18, DROP INDEX CONCURRENTLY accepts only one index name, cannot use CASCADE, cannot remove an index backing a UNIQUE or PRIMARY KEY constraint, cannot run inside a transaction block, and cannot be used for indexes on partitioned tables.
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