pgsql: Add IF NOT EXISTS processing to ALTER TABLE ADD COLUMN

Add IF NOT EXISTS processing to ALTER TABLE ADD COLUMN

Fabrízio de Royes Mello, reviewed by Payal Singh, Alvaro Herrera and
Michael Paquier.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2cd40adb85d1d5a7fd448721b326e73c1e318750

Modified Files
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doc/src/sgml/ref/alter_table.sgml         |    8 ++-
src/backend/commands/tablecmds.c          |   52 ++++++++++++-----
src/backend/parser/gram.y                 |   20 +++++++
src/test/regress/expected/alter_table.out |   89 +++++++++++++++++++++++++++++
src/test/regress/sql/alter_table.sql      |   31 ++++++++++
5 files changed, 182 insertions(+), 18 deletions(-)