pgsql: Add a WHEN clause to CREATE TRIGGER, allowing a boolean

Log Message:
-----------
Add a WHEN clause to CREATE TRIGGER, allowing a boolean expression to be
checked to determine whether the trigger should be fired.

For BEFORE triggers this is mostly a matter of spec compliance; but for AFTER
triggers it can provide a noticeable performance improvement, since queuing of
a deferred trigger event and re-fetching of the row(s) at end of statement can
be short-circuited if the trigger does not need to be fired.

Takahiro Itagaki, reviewed by KaiGai Kohei.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.210 -> r2.211)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.210&r2=2.211)
        trigger.sgml (r1.59 -> r1.60)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/trigger.sgml?r1=1.59&r2=1.60)
    pgsql/doc/src/sgml/ref:
        create_constraint.sgml (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_constraint.sgml?r1=1.20&r2=1.21)
        create_trigger.sgml (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_trigger.sgml?r1=1.51&r2=1.52)
    pgsql/src/backend/catalog:
        index.c (r1.323 -> r1.324)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.323&r2=1.324)
    pgsql/src/backend/commands:
        copy.c (r1.317 -> r1.318)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c?r1=1.317&r2=1.318)
        tablecmds.c (r1.305 -> r1.306)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.305&r2=1.306)
        trigger.c (r1.256 -> r1.257)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c?r1=1.256&r2=1.257)
    pgsql/src/backend/executor:
        execMain.c (r1.334 -> r1.335)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c?r1=1.334&r2=1.335)
        execQual.c (r1.254 -> r1.255)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execQual.c?r1=1.254&r2=1.255)
        execUtils.c (r1.165 -> r1.166)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execUtils.c?r1=1.165&r2=1.166)
        nodeModifyTable.c (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeModifyTable.c?r1=1.2&r2=1.3)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.451 -> r1.452)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.451&r2=1.452)
        equalfuncs.c (r1.373 -> r1.374)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.373&r2=1.374)
    pgsql/src/backend/parser:
        gram.y (r2.693 -> r2.694)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.693&r2=2.694)
    pgsql/src/backend/tcop:
        utility.c (r1.317 -> r1.318)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.317&r2=1.318)
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.314 -> r1.315)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.314&r2=1.315)
    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.552 -> r1.553)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.552&r2=1.553)
    pgsql/src/include/catalog:
        catversion.h (r1.550 -> r1.551)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.550&r2=1.551)
        pg_trigger.h (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_trigger.h?r1=1.35&r2=1.36)
        toasting.h (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/toasting.h?r1=1.9&r2=1.10)
    pgsql/src/include/commands:
        trigger.h (r1.77 -> r1.78)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h?r1=1.77&r2=1.78)
    pgsql/src/include/nodes:
        execnodes.h (r1.211 -> r1.212)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h?r1=1.211&r2=1.212)
        parsenodes.h (r1.415 -> r1.416)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.415&r2=1.416)
    pgsql/src/include/utils:
        rel.h (r1.115 -> r1.116)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/rel.h?r1=1.115&r2=1.116)
    pgsql/src/test/regress/expected:
        triggers.out (r1.28 -> r1.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/triggers.out?r1=1.28&r2=1.29)
    pgsql/src/test/regress/sql:
        triggers.sql (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/triggers.sql?r1=1.17&r2=1.18)