pgsql: Show sizes of FETCH queries as constants in pg_stat_statements

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: /messages/by-id/CAA5RZ0tA6LbHCg2qSS+KuM850BZC_+ZgHV7Ug6BXw22TNyF+MA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bee23ea4ddc46198c95a4e73a83f453c09e04bf8

Modified Files
--------------
contrib/pg_stat_statements/expected/cursors.out | 141 ++++++++++++++++++++-
.../pg_stat_statements/expected/level_tracking.out | 4 +-
contrib/pg_stat_statements/expected/utility.out | 2 +-
contrib/pg_stat_statements/sql/cursors.sql | 43 +++++++
src/backend/parser/gram.y | 50 ++++++--
src/include/nodes/parsenodes.h | 35 ++++-
src/tools/pgindent/typedefs.list | 1 +
7 files changed, 259 insertions(+), 17 deletions(-)