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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 system catalogs, and remove it with DROP INDEX. Hash indexes support equality lookups only, index one column, and cannot enforce uniqueness; confirm a hash index fits your workload before deploying it.
Create a hash index
Specify USING hash in the CREATE INDEX statement. If you omit the method, PostgreSQL creates a B-tree index instead. The index is created in the same schema as its table, so a descriptive name and a schema-qualified table reference help avoid ambiguity.
CREATE INDEX users_email_hash_idx
ON public.users USING hash (email);This creates a single-column, non-unique index on public.users.email. PostgreSQL hash indexes cannot be used to enforce uniqueness; use a suitable unique constraint or unique B-tree index when uniqueness is required.
IF NOT EXISTS can avoid an error if an index with the same name already exists, but it does not verify that the existing index has the requested table, column, or method. Inspect the definition before assuming it matches. See the PostgreSQL 18 CREATE INDEX documentation.
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Choose blocking or concurrent creation
A regular index build blocks writes to the table until it finishes, although reads can continue. For a live table, PostgreSQL offers concurrent creation:
CREATE INDEX CONCURRENTLY users_email_hash_idx
ON public.users USING hash (email);Concurrent creation allows ordinary inserts, updates, and deletes during the build, but takes longer: it performs two table scans and waits for relevant transactions. It cannot run inside a transaction block, and only one concurrent index build can run on a table at a time.
If a concurrent build fails, it can leave an invalid index. Such an index is ignored for queries but still adds overhead to table updates. Check its status, remove the invalid index and retry, or consider REINDEX INDEX CONCURRENTLY where appropriate. In PostgreSQL 18, concurrent creation is not supported as a single operation on a partitioned table; the documented approach is to build indexes concurrently on individual partitions and attach them through the supported procedure. Consult the PostgreSQL 18 CREATE INDEX documentation for the command’s full restrictions.
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Inspect indexes and confirm the method
In psql, di lists indexes, while di+ adds details such as disk size. To see the indexes and definitions associated with a table, use d public.users. An invalid index left by a failed concurrent build can appear as INVALID in the table display. The psql reference describes these commands.
For SQL-based inspection, the pg_indexes view provides the schema, table, index name, tablespace, and reconstructed definition:
SELECT schemaname, tablename, indexname, tablespace, indexdef
FROM pg_indexes
WHERE schemaname = 'public'
AND tablename = 'users'
ORDER BY indexname;To confirm the access method directly, join the index relation in pg_class to pg_am. This distinguishes a hash index from a B-tree even when the index name is unclear:
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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 query lists ordinary indexes in the public schema; add a table or index-name condition to narrow the results. A partitioned index parent has a different relation kind, so include it separately if you are inspecting partitioned indexes. The relevant catalog fields and view are documented in pg_class, pg_am, and pg_indexes.
Decide whether a hash index suits the query
A hash index is intended for equality predicates such as email = 'person@example.com'. It does not support range predicates such as email > 'm'. A B-tree supports equality and ordered or range comparisons, and can also be multicolumn or unique; a hash index is single-column and non-unique.
Hash-index entries store a four-byte hash value rather than the original indexed value. Because the stored value is not the original key, index scans are lossy and must recheck matching table rows. Hash indexes can participate in bitmap index scans. PostgreSQL notes that a hash index may be smaller than a B-tree for longer values such as UUIDs or URLs, but that does not establish a speed advantage. Bucket growth, overflow behavior, row distribution, and the number of rows mapping to buckets can affect performance.
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There is no universal performance winner. Compare the methods using the actual predicate, data distribution, value lengths, table size, insert and update pattern, and query selectivity. The PostgreSQL documentation describes these trade-offs in Hash Indexes and Index Types; it does not provide a current numeric benchmark proving hash indexes are faster.
Check the planner and measure representative work
First make sure table statistics are current when needed, then inspect the plan for the real query:
ANALYZE public.users;
EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM public.users WHERE email = 'person@example.com';ANALYZE refreshes planner statistics; whether it is needed depends on how current the table’s statistics are. EXPLAIN shows the planner’s chosen plan. EXPLAIN ANALYZE actually executes the query and reports actual rows and timing alongside estimates, so use it with care for statements that modify data or have side effects. Its measurements have overhead and exclude client network transfer; results on a small test table should not be assumed to predict behavior on a substantially different production workload. Do not force planner settings as proof that an index helps; evaluate natural plans and representative workload measurements. See the PostgreSQL 18 EXPLAIN documentation.
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Drop a hash index
Remove an index by name, including its schema when needed:
DROP INDEX public.users_email_hash_idx;Use IF EXISTS if a missing or previously removed index should produce a notice rather than an error:
DROP INDEX IF EXISTS public.users_email_hash_idx;The index owner must run the command. By default, RESTRICT refuses the drop if dependent objects exist. CASCADE recursively removes dependent objects, so use it only after reviewing what depends on the index.
Drop without blocking concurrent table access
On an active table, you can request a concurrent drop:
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PostgreSQL 18 command reference
| Task | Command | Key consideration |
|---|---|---|
| Create | CREATE INDEX users_email_hash_idx ON public.users USING hash (email); |
Equality only; one column; not unique. |
| Create during ordinary writes | CREATE INDEX CONCURRENTLY users_email_hash_idx ON public.users USING hash (email); |
Longer build, two scans, waits; no transaction block. |
| Inspect in psql | di+ or d public.users |
Check the definition and whether an index is invalid. |
| Drop | DROP INDEX public.users_email_hash_idx; |
Regular drop can block table access. |
| Drop during concurrent access | DROP INDEX CONCURRENTLY public.users_email_hash_idx; |
One index only; no transaction block, cascade, or partitioned-table index. |
These commands and operational details refer to PostgreSQL 18 documentation. PostgreSQL’s current documentation alias can move to a later release, so check the documentation matching your server version when applying them.
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