# Adding an Index
**Check name:** `AddIndexCheck`
**Lock type:** SHARE (blocks all writes)
## Bad
Creating an index without CONCURRENTLY acquires a SHARE lock, blocking all write operations (INSERT, UPDATE, DELETE) for the duration of the index build.
```sql
CREATE INDEX idx_users_email ON users(email);
CREATE UNIQUE INDEX idx_users_username ON users(username);
```
## Good
Use CONCURRENTLY to allow concurrent writes during the index build:
```sql
CREATE INDEX CONCURRENTLY idx_users_email ON users(email);
CREATE UNIQUE INDEX CONCURRENTLY idx_users_username ON users(username);
```
**Important:** CONCURRENTLY cannot run inside a transaction block.
**For Diesel migrations:** Add a `metadata.toml` file to your migration directory:
```toml
# migrations/2024_01_01_add_user_index/metadata.toml
run_in_transaction = false
```
**For SQLx migrations:** Add the no-transaction directive at the top of your migration file:
```sql
-- no-transaction
CREATE INDEX CONCURRENTLY idx_users_email ON users(email);
```