Index sequence(exemplar) only where exemplar=true.

sql/seqdef.sql

@@ -301,7 +301,7 @@ ON UPDATE CASCADE
 );
 
 CREATE UNIQUE INDEX i_s1 ON sequences(locus,md5(sequence));
-CREATE INDEX i_s2 ON sequences(exemplar);
+CREATE INDEX i_s2 ON sequences(exemplar) WHERE exemplar;
 GRANT SELECT,UPDATE,INSERT,DELETE ON sequences TO apache;
 
 CREATE TABLE sequence_extended_attributes (

sql/upgrade/seqdefdb_v1.14.sql

-CREATE INDEX i_s2 ON sequences(exemplar);
+CREATE INDEX i_s2 ON sequences(exemplar) WHERE exemplar;
 
 --Change owner of profile_members to apache (needed so that triggers can be disabled for batch upload).
 ALTER TABLE profile_members OWNER TO apache;