Show sizes of FETCH queries as constants in pg_stat_statements

Prior to this patch, every FETCH call would generate a unique queryId
with a different size specified.  Depending on the workloads, this could
lead to a significant bloat in pg_stat_statements, as repeatedly calling
a specific cursor would result in a new queryId each time.  For example,
FETCH 1 c1; and FETCH 2 c1; would produce different queryIds.

This patch improves the situation by normalizing the fetch size, so as
semantically similar statements generate the same queryId.  As a result,
statements like the below, which differ syntactically but have the same
effect, will now share a single queryId:
FETCH FROM c1
FETCH NEXT c1
FETCH 1 c1

In order to do a normalization based on the keyword used in FETCH,
FetchStmt is tweaked with a new FetchDirectionKeywords.  This matters
for "howMany", which could be set to a negative value depending on the
direction, and we want to normalize the queries with enough information
about the direction keywords provided, including RELATIVE, ABSOLUTE or
all the ALL variants.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0tA6LbHCg2qSS+KuM850BZC_+ZgHV7Ug6BXw22TNyF+MA@mail.gmail.com

1 files changed, 32 insertions, 3 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index ba12678d1cb..daa285ca62f 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -3422,15 +3422,44 @@ typedef enum FetchDirection
 	FETCH_RELATIVE,
 } FetchDirection;
 
+typedef enum FetchDirectionKeywords
+{
+	FETCH_KEYWORD_NONE = 0,
+	FETCH_KEYWORD_NEXT,
+	FETCH_KEYWORD_PRIOR,
+	FETCH_KEYWORD_FIRST,
+	FETCH_KEYWORD_LAST,
+	FETCH_KEYWORD_ABSOLUTE,
+	FETCH_KEYWORD_RELATIVE,
+	FETCH_KEYWORD_ALL,
+	FETCH_KEYWORD_FORWARD,
+	FETCH_KEYWORD_FORWARD_ALL,
+	FETCH_KEYWORD_BACKWARD,
+	FETCH_KEYWORD_BACKWARD_ALL,
+} FetchDirectionKeywords;
+
 #define FETCH_ALL	LONG_MAX
 
 typedef struct FetchStmt
 {
 	NodeTag		type;
 	FetchDirection direction;	/* see above */
-	long		howMany;		/* number of rows, or position argument */
-	char	   *portalname;		/* name of portal (cursor) */
-	bool		ismove;			/* true if MOVE */
+	/* number of rows, or position argument */
+	long		howMany pg_node_attr(query_jumble_ignore);
+	/* name of portal (cursor) */
+	char	   *portalname;
+	/* true if MOVE */
+	bool		ismove;
+
+	/*
+	 * Set when a direction_keyword (e.g., FETCH FORWARD) is used, to
+	 * distinguish it from a numeric variant (e.g., FETCH 1) for the purpose
+	 * of query jumbling.
+	 */
+	FetchDirectionKeywords direction_keyword;
+
+	/* token location, or -1 if unknown */
+	ParseLoc	location pg_node_attr(query_jumble_location);
 } FetchStmt;
 
 /* ----------------------