Cache Hint

A query cache hint can be used to:

Indicate that a user query is eligible for result set caching and set the cache entry memory preference, time to live, etc;
Set the cached view memory preference, time to live, or updatability;
Indicate that a virtual procedure should be cachable and set the cache entry memory preference, time to live, etc.:
/*+ cache[([pref_mem] [ttl:n] [updatable] [scope:session|user|vdb])]*/ sql ...
The cache hint should appear at the beginning of the SQL. It can appear as any one of the leading comments, and will not have any effect on INSERT/UPDATE/DELETE statements or INSTEAD OF TRIGGERS.

The parameters in square brackets are optional, and, if present, they mean the following:

Parameter	Description
`pref_mem`	Indicates that the cached results should preferably remain in memory. The results may still be paged out based upon memory pressure. Please be careful to not over use the `pref_mem` option. The memory preference is implemented with Java soft references. While soft references are effective at preventing out of memory conditions, too much memory held by soft references can limit the effective working memory. Consult your JVM options for clearing soft references if you need to tune their behaviour
`ttl:n`	Indicates the time to live value in milliseconds. The default value for result set caching is the default expiration for the corresponding `Infinispan` cache. There is no default time to live for cached views
`updatable`	Indicates that the cached results can be updated. Defaults to `FALSE` for cached views and to `TRUE` for result set cache entries
`scope`	There are three different cache scopes: `session` - cached only for current session; `user` - cached for any session by the current user; `vdb` - cached for any user connected to the same vdb. For cached queries, the presence of the scope overrides the computed scope. Cached views, on the other hand, default to the vdb scope. For cached views, explicitly setting the session or user scopes will result in a non-replicated session scoped cached view

Example

In this example, the cached result of the query will preferably be stored in memory for 1 minute:

CODE

/*+ cache(pref_mem ttl:60000) */ SELECT COUNT(*) FROM (CALL "s3_no_path1.listFiles"(
    "pathAndPattern" => '/xxx/*'
)) AS a ;;

Limitations

The form of the query hint must be matched exactly for the hint to have effect. For a user query, if the hint is not specified correctly, e.g. /*+ cach(pref_mem) */, it will not be used by the engine nor will there be any information in the log.

OPTION NOCACHE

Individual queries may override the use of cached results by specifying OPTION NOCACHE on the query. You can specify 0 or more fully qualified view or procedure names to exclude using their cached results. If no names are specified, cached results will not be used transitively.

Full NOCACHE

In this case, no results will be used at all:

CODE

SELECT * from vg1, vg2, vg3 WHERE … OPTION NOCACHE

Specific NOCACHE

In this example, only the vg1 and vg3 caches will be skipped, vg2 or any cached results nested under vg1 and vg3 will be used:

CODE

SELECT * from vg1, vg2, vg3 WHERE … OPTION NOCACHE vg1, vg3

OPTION NOCACHE may be specified in procedure or view definitions. In that way, transformations can specify to always use real-time data obtained directly from sources.