registerSnapshotEngine

registerSnapshotEngine(dbName, tableName, keyColumnName)

dbName is a string indicating the name of a distributed database.

tableName is a string indicating the name of a table.

keyColumnName is a string indicating the name of a column.

Register a snapshot engine for a distributed table. After registering the snapshot engine, we can get the most recent row of each group specified by keyColumnName with the following SQL statement:

select [HINT_SNAPSHOT] * from loadTable(dbName,tableName).

Please note that this command can be executed in single node mode. If used in a cluster, we cannot specify where conditions in the SQL statement.

Only one snapshot engine can be registered for a DFS table. After a system restart, we need to register the snapshot engine again.

Use command unregisterSnapshotEngine to unregister a snapshot engine.

Create a partitioned table:

db1=database("",VALUE,2018.09.01..2018.09.30)
db2=database("",VALUE,`AAPL`MSFT`MS`C)
db=database("dfs://compoDB",COMPO,[db1,db2])
t=table(1:0,`date`sym`val,[DATE,SYMBOL,DOUBLE])
pt=db.createPartitionedTable(t,`pt,`date`sym);

registerSnapshotEngine("dfs://compoDB","pt","sym");

def writeData(batch){
    pt=loadTable("dfs://compoDB","pt")
    tmp=table(batch:0,`date`sym`val,[DATE,SYMBOL,DOUBLE])
    dates=take(2018.09.01..2018.09.30,batch)
    syms=take(`AAPL`MSFT`MS`C,batch)
    vals=rand(100.0,batch)
    insert into tmp values(dates,syms,vals)
    pt.append!(tmp)
}

writeData(1000);

select [HINT_SNAPSHOT] * from loadTable("dfs://compoDB","pt");

writeData(1000);
select [HINT_SNAPSHOT] * from loadTable("dfs://compoDB","pt");