Thread: pgsql: Add unchangeable GUC "variables" segment_size, wal_block_size,

Log Message:
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Add unchangeable GUC "variables" segment_size, wal_block_size, and
wal_segment_size to make those configuration parameters available to clients,
in the same way that block_size was previously exposed.  Bernd Helmle, with
comments from Abhijit Menon-Sen and some further tweaking by me.

Modified Files:
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    pgsql/doc/src/sgml:
        config.sgml (r1.182 -> r1.183)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.182&r2=1.183)
    pgsql/src/backend/utils/misc:
        guc.c (r1.463 -> r1.464)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.463&r2=1.464)