Allow HOT updates for some expression indexes

If the value of an index expression is unchanged after UPDATE,
allow HOT updates where previously we disallowed them, giving
a significant performance boost in those cases.

Particularly useful for indexes such as JSON->>field where the
JSON value changes but the indexed value does not.

Submitted as "surjective indexes" patch, now enabled by use
of new "recheck_on_update" parameter.

Author: Konstantin Knizhnik
Reviewer: Simon Riggs, with much wordsmithing and some cleanup

1 files changed, 44 insertions, 1 deletions

diff --git a/src/backend/access/common/reloptions.c b/src/backend/access/common/reloptions.c
index 46276ceff1c..35c09987adb 100644
--- a/src/backend/access/common/reloptions.c
+++ b/src/backend/access/common/reloptions.c
@@ -131,6 +131,15 @@ static relopt_bool boolRelOpts[] =
 	},
 	{
 		{
+			"recheck_on_update",
+			"Recheck functional index expression for changed value after update",
+			RELOPT_KIND_INDEX,
+			ShareUpdateExclusiveLock	/* since only applies to later UPDATEs */
+		},
+		true
+	},
+	{
+		{
 			"security_barrier",
 			"View acts as a row security barrier",
 			RELOPT_KIND_VIEW,
@@ -1310,7 +1319,7 @@ fillRelOptions(void *rdopts, Size basesize,
 				break;
 			}
 		}
-		if (validate && !found)
+		if (validate && !found && options[i].gen->kinds != RELOPT_KIND_INDEX)
 			elog(ERROR, "reloption \"%s\" not found in parse table",
 				 options[i].gen->name);
 	}
@@ -1467,6 +1476,40 @@ index_reloptions(amoptions_function amoptions, Datum reloptions, bool validate)
 }
 
 /*
+ * Parse generic options for all indexes.
+ *
+ *	reloptions	options as text[] datum
+ *	validate	error flag
+ */
+bytea *
+index_generic_reloptions(Datum reloptions, bool validate)
+{
+	int              numoptions;
+	GenericIndexOpts *idxopts;
+	relopt_value     *options;
+	static const relopt_parse_elt tab[] = {
+		{"recheck_on_update", RELOPT_TYPE_BOOL, offsetof(GenericIndexOpts, recheck_on_update)}
+	};
+
+	options = parseRelOptions(reloptions, validate,
+							  RELOPT_KIND_INDEX,
+							  &numoptions);
+
+	/* if none set, we're done */
+	if (numoptions == 0)
+		return NULL;
+
+	idxopts = allocateReloptStruct(sizeof(GenericIndexOpts), options, numoptions);
+
+	fillRelOptions((void *)idxopts, sizeof(GenericIndexOpts), options, numoptions,
+				   validate, tab, lengthof(tab));
+
+	pfree(options);
+
+	return (bytea*) idxopts;
+}
+
+/*
  * Option parser for attribute reloptions
  */
 bytea *