Use hash of the cache key to limit in-memory heap overhead for large search requests

docs/configuration/node-config.md

@@ -307,7 +307,7 @@ This section contains the configuration options for a Searcher.
 | `aggregation_bucket_limit` | Determines the maximum number of buckets returned to the client. | `65000` |
 | `fast_field_cache_capacity` | Fast field in memory cache capacity on a Searcher. If your filter by dates, run aggregations, range queries, or even for tracing, it might worth increasing this parameter. The [metrics](../reference/metrics.md) starting by `quickwit_cache_fastfields_cache` can help you make an informed choice when setting this value. | `1G` |
 | `split_footer_cache_capacity` | Split footer in memory cache (it is essentially the hotcache) capacity on a Searcher.| `500M` |
-| `partial_request_cache_capacity` | Partial request in memory cache capacity on a Searcher. Cache intermediate state for a request, possibly making subsequent requests faster. It can be disabled by setting the size to `0`. | `64M` |
+| `partial_request_cache_capacity` | Partial request in memory cache capacity on a Searcher. Cache intermediate state for a request, possibly making subsequent requests faster. Entries are keyed by a 128-bit hash of the request, with a negligeable probability of cache collision. It can be disabled by setting the size to `0`. | `64M` |
 | `max_num_concurrent_split_searches` | Maximum number of concurrent split search requests running on a Searcher. | `100` |
 | `split_cache` | Searcher split cache configuration options defined in the section below. Cache disabled if unspecified. | |
 | `request_timeout_secs` | The time before a search request is cancelled. This should match the timeout of the stack calling into quickwit if there is one set.  | `30` |

quickwit/quickwit-search/Cargo.toml

@@ -26,6 +26,7 @@ postcard = { workspace = true }
 prost = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+siphasher = { workspace = true }
 tantivy = { workspace = true }
 tantivy-fst = { workspace = true }
 thiserror = { workspace = true }