pgsql: Reject consecutive element patterns of same kind

Reject consecutive element patterns of same kind

Adding an implicit empty vertex pattern when a path pattern starts or
ends with an edge pattern or when two consecutive edge patterns appear
in the pattern is not supported right now.  Prohibit such path
patterns.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Henson Choi <assam258@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/72a23702-6d96-4103-a54b-057c2352e885%2540eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/720f0f89d696a5c12817359b5c0f3da8e7ebdfe8

Modified Files
--------------
src/backend/parser/parse_graphtable.c     | 48 +++++++++++++++++++++++++++----
src/backend/rewrite/rewriteGraphTable.c   | 10 +++++++
src/test/regress/expected/graph_table.out | 19 ++++++++++++
src/test/regress/sql/graph_table.sql      |  6 ++++
4 files changed, 78 insertions(+), 5 deletions(-)