feat(cubestore): Support EXPLAIN for meta queries

Author: ovr

rust/cubestore/cubestore/src/queryplanner/pretty_printers.rs

@@ -30,7 +30,7 @@ use crate::queryplanner::serialized_plan::{IndexSnapshot, RowRange};
 use crate::queryplanner::tail_limit::TailLimitExec;
 use crate::queryplanner::topk::ClusterAggregateTopK;
 use crate::queryplanner::topk::{AggregateTopKExec, SortColumn};
-use crate::queryplanner::CubeTableLogical;
+use crate::queryplanner::{CubeTableLogical, InfoSchemaTableProvider};
 use datafusion::cube_ext::join::CrossJoinExec;
 use datafusion::cube_ext::joinagg::CrossJoinAggExec;
 use datafusion::cube_ext::rolling::RollingWindowAggExec;
@@ -257,6 +257,8 @@ fn pp_source(t: &dyn TableProvider) -> String {
         format!("CubeTable(index: {})", pp_index(t.index_snapshot()))
     } else if let Some(t) = t.as_any().downcast_ref::<InlineTableProvider>() {
         format!("InlineTableProvider(data: {} rows)", t.get_data().len())
+    } else if t.as_any().downcast_ref::<InfoSchemaTableProvider>().is_some() {
+        "InfoSchemaTableProvider".to_string()
     } else {
         panic!("unknown table provider");
     }

rust/cubestore/cubestore/src/sql/mod.rs

@@ -461,9 +461,22 @@ impl SqlServiceImpl {
                 };
                 Ok(res)
             }
-            _ => Err(CubeError::user(
-                "Explain not supported for selects from system tables".to_string(),
-            )),
+            QueryPlan::Meta(logical_plan) => {
+                if !analyze {
+                    Ok(DataFrame::new(
+                        vec![Column::new(
+                            "logical plan".to_string(),
+                            ColumnType::String,
+                            0,
+                        )],
+                        vec![Row::new(vec![TableValue::String(pp_plan(&logical_plan))])],
+                    ))
+                } else {
+                    Err(CubeError::user(
+                        "EXPLAIN ANALYZE is not supported for selects from system tables".to_string(),
+                    ))
+                }
+            }
         }?;
         Ok(Arc::new(res))
     }