Limit the optimizer mdcache default size to 16MB.
Limit the optimizer mdcache default size to 16MB.

Based on the experiments with tpch and tpcds, we found that 16MB would
be the optimal size of mdcache.
[#96490982].

ashwinstar/gpdb,adam8157/gpdb,edespino/gpdb,rvs/gpdb,rvs/gpdb,edespino/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,Chibin/gpdb,Quikling/gpdb,lisakowen/gpdb,rvs/gpdb,janebeckman/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,janebeckman/gpdb,cjcjameson/gpdb,lisakowen/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,janebeckman/gpdb,Quikling/gpdb,xinzweb/gpdb,cjcjameson/gpdb,rvs/gpdb,yuanzhao/gpdb,adam8157/gpdb,xinzweb/gpdb,janebeckman/gpdb,janebeckman/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,Quikling/gpdb,cjcjameson/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,lisakowen/gpdb,cjcjameson/gpdb,edespino/gpdb,lisakowen/gpdb,Quikling/gpdb,ashwinstar/gpdb,Quikling/gpdb,yuanzhao/gpdb,50wu/gpdb,rvs/gpdb,Quikling/gpdb,greenplum-db/gpdb,rvs/gpdb,kaknikhil/gpdb,kaknikhil/gpdb,edespino/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,lisakowen/gpdb,50wu/gpdb,rvs/gpdb,edespino/gpdb,edespino/gpdb,Chibin/gpdb,ashwinstar/gpdb,janebeckman/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,Chibin/gpdb,kaknikhil/gpdb,edespino/gpdb,cjcjameson/gpdb,greenplum-db/gpdb,Quikling/gpdb,greenplum-db/gpdb,xinzweb/gpdb,Chibin/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Chibin/gpdb,Quikling/gpdb,ashwinstar/gpdb,xinzweb/gpdb,edespino/gpdb,50wu/gpdb,janebeckman/gpdb,yuanzhao/gpdb,adam8157/gpdb,edespino/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,50wu/gpdb,yuanzhao/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,yuanzhao/gpdb,rvs/gpdb,kaknikhil/gpdb,Chibin/gpdb,jmcatamney/gpdb,Chibin/gpdb,cjcjameson/gpdb,Quikling/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,edespino/gpdb,Chibin/gpdb,janebeckman/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,adam8157/gpdb,lisakowen/gpdb,Quikling/gpdb,rvs/gpdb,50wu/gpdb,janebeckman/gpdb,ashwinstar/gpdb,50wu/gpdb,kaknikhil/gpdb,rvs/gpdb,janebeckman/gpdb,50wu/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,Chibin/gpdb,kaknikhil/gpdb,Chibin/gpdb