Improve comments about semijoin implementation strategy, per a question from Robert Haas.
Improve comments about semijoin implementation strategy, per a question
from Robert Haas.

yazun/postgres-xl,arcivanov/postgres-xl,lisakowen/gpdb,yuanzhao/gpdb,Chibin/gpdb,oberstet/postgres-xl,yuanzhao/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,Chibin/gpdb,ovr/postgres-xl,xinzweb/gpdb,Chibin/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,Chibin/gpdb,50wu/gpdb,snaga/postgres-xl,greenplum-db/gpdb,edespino/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,Chibin/gpdb,jmcatamney/gpdb,yazun/postgres-xl,yuanzhao/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,Chibin/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,postmind-net/postgres-xl,50wu/gpdb,Chibin/gpdb,oberstet/postgres-xl,lisakowen/gpdb,snaga/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,xinzweb/gpdb,greenplum-db/gpdb,tpostgres-projects/tPostgres,zeroae/postgres-xl,lisakowen/gpdb,oberstet/postgres-xl,yazun/postgres-xl,edespino/gpdb,Chibin/gpdb,pavanvd/postgres-xl,edespino/gpdb,arcivanov/postgres-xl,adam8157/gpdb,Chibin/gpdb,yazun/postgres-xl,Chibin/gpdb,yuanzhao/gpdb,lisakowen/gpdb,postmind-net/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,yazun/postgres-xl,techdragon/Postgres-XL,arcivanov/postgres-xl,kmjungersen/PostgresXL,xinzweb/gpdb,xinzweb/gpdb,edespino/gpdb,Postgres-XL/Postgres-XL,edespino/gpdb,zeroae/postgres-xl,tpostgres-projects/tPostgres,lisakowen/gpdb,adam8157/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,ashwinstar/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,greenplum-db/gpdb,snaga/postgres-xl,50wu/gpdb,ovr/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,50wu/gpdb,xinzweb/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,edespino/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,postmind-net/postgres-xl,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,tpostgres-projects/tPostgres,zeroae/postgres-xl,postmind-net/postgres-xl,yuanzhao/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,yuanzhao/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,kmjungersen/PostgresXL,postmind-net/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,kmjungersen/PostgresXL,techdragon/Postgres-XL,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,edespino/gpdb,adam8157/gpdb,ovr/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,tpostgres-projects/tPostgres,edespino/gpdb,pavanvd/postgres-xl,yuanzhao/gpdb,tpostgres-projects/tPostgres,snaga/postgres-xl,ovr/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,jmcatamney/gpdb