pgsql: interval: tighten precision specification

interval:  tighten precision specification

interval precision can only be specified after the "interval" keyword if
no units are specified.

Previously we incorrectly checked the units to see if the precision was
legal, causing confusion.

Report by Alvaro Herrera

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/417f92484da8c9a124e7eece8dbfb71f688a0104

Modified Files
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src/backend/parser/gram.y              |   54 ++++++--------------------------
src/test/regress/expected/interval.out |   10 ------
src/test/regress/sql/interval.sql      |    2 --
3 files changed, 9 insertions(+), 57 deletions(-)