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, 41 insertions, 9 deletions

diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 50f53159d58..1c11b235aa6 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -7477,6 +7477,8 @@ fetch_args:	cursor_name
 					n->portalname = $1;
 					n->direction = FETCH_FORWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_NONE;
 					$$ = (Node *) n;
 				}
 			| from_in cursor_name
@@ -7486,6 +7488,19 @@ fetch_args:	cursor_name
 					n->portalname = $2;
 					n->direction = FETCH_FORWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_NONE;
+					$$ = (Node *) n;
+				}
+			| SignedIconst opt_from_in cursor_name
+				{
+					FetchStmt *n = makeNode(FetchStmt);
+
+					n->portalname = $3;
+					n->direction = FETCH_FORWARD;
+					n->howMany = $1;
+					n->location = @1;
+					n->direction_keyword = FETCH_KEYWORD_NONE;
 					$$ = (Node *) n;
 				}
 			| NEXT opt_from_in cursor_name
@@ -7495,6 +7510,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_FORWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_NEXT;
 					$$ = (Node *) n;
 				}
 			| PRIOR opt_from_in cursor_name
@@ -7504,6 +7521,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_BACKWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_PRIOR;
 					$$ = (Node *) n;
 				}
 			| FIRST_P opt_from_in cursor_name
@@ -7513,6 +7532,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_ABSOLUTE;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_FIRST;
 					$$ = (Node *) n;
 				}
 			| LAST_P opt_from_in cursor_name
@@ -7522,6 +7543,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_ABSOLUTE;
 					n->howMany = -1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_LAST;
 					$$ = (Node *) n;
 				}
 			| ABSOLUTE_P SignedIconst opt_from_in cursor_name
@@ -7531,6 +7554,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_ABSOLUTE;
 					n->howMany = $2;
+					n->location = @2;
+					n->direction_keyword = FETCH_KEYWORD_ABSOLUTE;
 					$$ = (Node *) n;
 				}
 			| RELATIVE_P SignedIconst opt_from_in cursor_name
@@ -7540,15 +7565,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_RELATIVE;
 					n->howMany = $2;
-					$$ = (Node *) n;
-				}
-			| SignedIconst opt_from_in cursor_name
-				{
-					FetchStmt *n = makeNode(FetchStmt);
-
-					n->portalname = $3;
-					n->direction = FETCH_FORWARD;
-					n->howMany = $1;
+					n->location = @2;
+					n->direction_keyword = FETCH_KEYWORD_RELATIVE;
 					$$ = (Node *) n;
 				}
 			| ALL opt_from_in cursor_name
@@ -7558,6 +7576,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_FORWARD;
 					n->howMany = FETCH_ALL;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_ALL;
 					$$ = (Node *) n;
 				}
 			| FORWARD opt_from_in cursor_name
@@ -7567,6 +7587,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_FORWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_FORWARD;
 					$$ = (Node *) n;
 				}
 			| FORWARD SignedIconst opt_from_in cursor_name
@@ -7576,6 +7598,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_FORWARD;
 					n->howMany = $2;
+					n->location = @2;
+					n->direction_keyword = FETCH_KEYWORD_FORWARD;
 					$$ = (Node *) n;
 				}
 			| FORWARD ALL opt_from_in cursor_name
@@ -7585,6 +7609,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_FORWARD;
 					n->howMany = FETCH_ALL;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_FORWARD_ALL;
 					$$ = (Node *) n;
 				}
 			| BACKWARD opt_from_in cursor_name
@@ -7594,6 +7620,8 @@ fetch_args:	cursor_name
 					n->portalname = $3;
 					n->direction = FETCH_BACKWARD;
 					n->howMany = 1;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_BACKWARD;
 					$$ = (Node *) n;
 				}
 			| BACKWARD SignedIconst opt_from_in cursor_name
@@ -7603,6 +7631,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_BACKWARD;
 					n->howMany = $2;
+					n->location = @2;
+					n->direction_keyword = FETCH_KEYWORD_BACKWARD;
 					$$ = (Node *) n;
 				}
 			| BACKWARD ALL opt_from_in cursor_name
@@ -7612,6 +7642,8 @@ fetch_args:	cursor_name
 					n->portalname = $4;
 					n->direction = FETCH_BACKWARD;
 					n->howMany = FETCH_ALL;
+					n->location = -1;
+					n->direction_keyword = FETCH_KEYWORD_BACKWARD_ALL;
 					$$ = (Node *) n;
 				}
 		;