From the documentation:
* `never`: When there is an active transaction, no data will be returned
from the cache, but all requests will always be sent to the backend.
The cache will be populated inside _explicitly_ read-only transactions.
Inside transactions that are not explicitly read-only, the cache will
be populated _until_ the first non-SELECT statement.
* `read_only_transactions`: The cache will be used and populated inside
_explicitly_ read-only transactions. Inside transactions that are not
explicitly read-only, the cache will be populated, but not used
_until_ the first non-SELECT statement.
* `all_transactions`: The cache will be used and populated inside
_explicitly_ read-only transactions. Inside transactions that are not
explicitly read-only, the cache will be used and populated _until_ the
first non-SELECT statement.
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