Improve similarity search performance

This commit is inspired by the change of pg_trgm: 25bfa7e.

diff --git a/bigm_gin.c b/bigm_gin.c
index 6ca4a45..27fa76a 100644 (file)
--- a/bigm_gin.c
+++ b/bigm_gin.c
@@ -254,15 +254,25 @@ gin_bigm_consistent(PG_FUNCTION_ARGS)
                                if (check[i])
                                        ntrue++;
                        }
-#ifdef DIVUNION
-                       res = (nkeys == ntrue) ? true :
-                               ((((((float4) ntrue) / ((float4) (nkeys - ntrue)))) >=
-                                 (float4) bigm_similarity_limit) ? true : false);
-#else
+
+                       /*--------------------
+                        * If DIVUNION is defined then similarity formula is:
+                        * c / (len1 + len2 - c)
+                        * where c is number of common bigrams and it stands as ntrue in
+                        * this code.  Here we don't know value of len2 but we can assume
+                        * that c (ntrue) is a lower bound of len2, so upper bound of
+                        * similarity is:
+                        * c / (len1 + c - c)  => c / len1
+                        * If DIVUNION is not defined then similarity formula is:
+                        * c / max(len1, len2)
+                        * And again, c (ntrue) is a lower bound of len2, but c <= len1
+                        * just by definition and, consequently, upper bound of
+                        * similarity is just c / len1.
+                        * So, independly on DIVUNION the upper bound formula is the same.
+                        */
                        res = (nkeys == 0) ? false :
                                ((((((float4) ntrue) / ((float4) nkeys))) >=
                                  (float4) bigm_similarity_limit) ? true : false);
-#endif
                        break;
                default:
                        elog(ERROR, "unrecognized strategy number: %d", strategy);
@@ -318,15 +328,14 @@ gin_bigm_triconsistent(PG_FUNCTION_ARGS)
                                if (check[i] != GIN_FALSE)
                                        ntrue++;
                        }
-#ifdef DIVUNION
-                       res = (nkeys == ntrue) ? GIN_MAYBE :
-                               (((((float4) ntrue) / ((float4) (nkeys - ntrue))) >=
-                                 (float4) bigm_similarity_limit) ? GIN_MAYBE : GIN_FALSE);
-#else
+
+                       /*
+                        * See comment in gin_bigm_consistent() about upper bound formula
+                        */
                        res = (nkeys == 0) ? GIN_FALSE :
                                (((((float4) ntrue) / ((float4) nkeys)) >=
                                  (float4) bigm_similarity_limit) ? GIN_MAYBE : GIN_FALSE);
-#endif
+
                        if (res != GIN_FALSE && !bigm_enable_recheck)
                                res = GIN_TRUE;
                        break;