I had a delay in querying Stardog and Postgres of about a half hour (had to take a call). Things were working. After the delay ... Studio (even after restart) is returning the error ...
000012: Unable to execute virtual graph query. SQL string: SELECT "xx", "yy", "zz" FROM "schema"."tableName" WHERE "zz" = 'ABC' AND "yy" = 'george'
stardog.log indicates that ...
WARN 2020-05-28 16:23:27,716 [stardog-user-16] com.complexible.stardog.protocols.http.server.StardogUndertowErrorHandler:accept(64): Unexpected exception was handled by the server org.postgresql.util.PSQLException: This connection has been closed. at org.postgresql.jdbc.PgConnection.checkClosed(PgConnection.java:857) ~[postgresql-42.2.12.jar:42.2.12] at org.postgresql.jdbc.PgConnection.createStatement(PgConnection.java:1756) ~[postgresql-42.2.12.jar:42.2.12]
I had to STOP the Stardog server and restart it to re-establish a connection. Then, things worked. I can't stop my Stardog server in production.