sparse: make the default value of refine_factor to 1

Because of the removal of forward index, the current refine searching is
slower than before. To prevent performance degradation when using a
non-zero value of drop_ratio_search, set the default value of
refine_factor to 1.

Signed-off-by: Shawn Wang <[email protected]>

src/index/sparse/sparse_inverted_index_config.h

@@ -38,9 +38,19 @@ class SparseInvertedIndexConfig : public BaseConfig {
             .for_search()
             .for_range_search()
             .for_iterator();
+        /**
+         * refine_factor is used for approximate search.
+         * refine_factor == 1 means no refinement, and is the default value.
+         * refine_factor > 1 means refinement. The larger the value, the more
+         * accurate the approximate result will be, but the slower the
+         * performance.
+         * Be aware that if you opt to use a large drop_ratio_search, it is
+         * necessary for you to manually modify this value.
+         */
         KNOWHERE_CONFIG_DECLARE_FIELD(refine_factor)
             .description("refine factor")
-            .set_default(10)
+            .set_default(1)
+            .set_range(1, 10, true, true)
             .for_search()
             .for_range_search();
         /**