build a Seq of used TiColumnRef from AttributeSet and bound them to source table
build a Seq of used TiColumnRef from AttributeSet and bound them to source table
AttributeSet containing projects w/ or w/o filters
source TiDBRelation
a Seq of TiColumnRef extracted
CHECK Spark org.apache.spark.sql.Strategy
TODO: Too many hacks here since we hijack the planning but we don't have full control over planning stage We cannot pass context around during planning so a re-extract needed for push-down since a plan tree might contain Join which causes a single tree have multiple plans to push-down