There should be 2 kinds of ScanNode:
OlapScanNode
ExternalScanNode
The Backends used for ExternalScanNode should be controlled by FederationBackendPolicy.
But currently, only FileScanNode is controlled by FederationBackendPolicy, other scan node such as MysqlScanNode,
JdbcScanNode will use Mix Backend even if we enable and prefer to use Compute Backend.
In this PR, I modified the hierarchy of ExternalScanNode, the new hierarchy is:
ScanNode
OlapScanNode
SchemaScanNode
ExternalScanNode
MetadataScanNode
DataGenScanNode
EsScanNode
OdbcScanNode
MysqlScanNode
JdbcScanNode
FileScanNode
FileLoadScanNode
FileQueryScanNode
MaxComputeScanNode
IcebergScanNode
TVFScanNode
HiveScanNode
HudiScanNode
And previously, the BackendPolicy is the member of FileScanNode, now I moved it to the ExternalScanNode.
So that all subtype ExternalScanNode can use BackendPolicy to choose Compute Backend to execute the query.
All all ExternalScanNode should implement the abstract method createScanRangeLocations().
For scan node like jdbc scan node/mysql scan node, the scan range locations will be selected randomly from
compute node(if preferred).
And for compute node selection. If all scan nodes are external scan nodes, and prefer_compute_node_for_external_table
is set to true, the BE for this query will only select compute nodes.
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