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EXPLAIN Syntax

Hive provides an EXPLAIN command that shows the execution plan for a query. The syntax for this statement is as follows:

EXPLAIN [EXTENDED|DEPENDENCY|AUTHORIZATION] query

AUTHORIZATION is supported from HIVE-0.14.0 via HIVE-5961

The use of EXTENDED in the EXPLAIN statement produces extra information about the operators in the plan. This is typically physical information like file names.

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As an example, consider the following EXPLAIN query:

EXPLAIN
FROM src INSERT OVERWRITE TABLE dest_g1 SELECT src.key, sum(substr(src.value,4)) GROUP BY src.key;

The output of this statement contains the following parts:

• The Abstract Syntax Tree

  Code Block
  ABSTRACT SYNTAX TREE: (TOK_QUERY (TOK_FROM (TOK_TABREF src)) (TOK_INSERT (TOK_DESTINATION (TOK_TAB dest_g1)) (TOK_SELECT (TOK_SELEXPR (TOK_COLREF src key)) (TOK_SELEXPR (TOK_FUNCTION sum (TOK_FUNCTION substr (TOK_COLREF src value) 4)))) (TOK_GROUPBY (TOK_COLREF src key))))

• The Dependency Graph

  Code Block
  STAGE DEPENDENCIES: Stage-1 is a root stage Stage-2 depends on stages: Stage-1 Stage-0 depends on stages: Stage-2

  This shows that Stage-1 is the root stage, Stage-2 is executed after Stage-1 is done and Stage-0 is executed after Stage-2 is done.

• The plans of each Stage

  Code Block

  STAGE PLANS: Stage: Stage-1 Map Reduce Alias -> Map Operator Tree: src Reduce Output Operator key expressions: expr: key type: string sort order: + Map-reduce partition columns: expr: rand() type: double tag: -1 value expressions: expr: substr(value, 4) type: string Reduce Operator Tree: Group By Operator aggregations: expr: sum(UDFToDouble(VALUE.0)) keys: expr: KEY.0 type: string mode: partial1 File Output Operator compressed: false table: input format: org.apache.hadoop.mapred.SequenceFileInputFormat output format: org.apache.hadoop.mapred.SequenceFileOutputFormat name: binary_table Stage: Stage-2 Map Reduce Alias -> Map Operator Tree: /tmp/hive-zshao/67494501/106593589.10001 Reduce Output Operator key expressions: expr: 0 type: string sort order: + Map-reduce partition columns: expr: 0 type: string tag: -1 value expressions: expr: 1 type: double Reduce Operator Tree: Group By Operator aggregations: expr: sum(VALUE.0) keys: expr: KEY.0 type: string mode: final Select Operator expressions: expr: 0 type: string expr: 1 type: double Select Operator expressions: expr: UDFToInteger(0) type: int expr: 1 type: double File Output Operator compressed: false table: input format: org.apache.hadoop.mapred.TextInputFormat output format: org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat serde: org.apache.hadoop.hive.serde2.dynamic_type.DynamicSerDe name: dest_g1 Stage: Stage-0 Move Operator tables: replace: true table: input format: org.apache.hadoop.mapred.TextInputFormat output format: org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat serde: org.apache.hadoop.hive.serde2.dynamic_type.DynamicSerDe name: dest_g1

  In this example there are 2 map/reduce stages (Stage-1 and Stage-2) and 1 File System related stage (Stage-0). Stage-0 basically moves the results from a temporary directory to the directory corresponding to the table dest_g1.

A map/reduce stage itself comprises of 2 parts:

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The use of DEPENDENCY in the EXPLAIN statement produces extra information about the inputs in the plan. It shows various attributes for the inputs. For example, for a query like:

EXPLAIN DEPENDENCY
  SELECT key, count(1) FROM srcpart WHERE ds IS NOT NULL GROUP BY key

the following output is produced:

{"input_partitions":[{"partitionName":"default<at:var at:name="srcpart" />ds=2008-04-08/hr=11"},{"partitionName":"default<at:var at:name="srcpart" />ds=2008-04-08/hr=12"},{"partitionName":"default<at:var at:name="srcpart" />ds=2008-04-09/hr=11"},{"partitionName":"default<at:var at:name="srcpart" />ds=2008-04-09/hr=12"}],"input_tables":[{"tablename":"default@srcpart","tabletype":"MANAGED_TABLE"}]}

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The dependencies show the parents in case a table is accessed via a view. Consider the following queries:

CREATE VIEW V1 AS SELECT key, value from src;
EXPLAIN DEPENDENCY SELECT * FROM V1;

The following output is produced:

{"input_partitions":[],"input_tables":[{"tablename":"default@v1","tabletype":"VIRTUAL_VIEW"},{"tablename":"default@src","tabletype":"MANAGED_TABLE","tableParents":"[default@v1]"}]}

...

All the outputs are shown if a table is being accessed via multiple parents.

CREATE VIEW V2 AS SELECT ds, key, value FROM srcpart WHERE ds IS NOT NULL;
CREATE VIEW V4 AS
  SELECT src1.key, src2.value as value1, src3.value as value2
  FROM V1 src1 JOIN V2 src2 on src1.key = src2.key JOIN src src3 ON src2.key = src3.key;
EXPLAIN DEPENDENCY SELECT * FROM V4;

The following output is produced.

{"input_partitions":[{"partitionParents":"[default@v2]","partitionName":"default<at:var at:name="srcpart" />ds=2008-04-08/hr=11"},{"partitionParents":"[default@v2]","partitionName":"default<at:var at:name="srcpart" />ds=2008-04-08/hr=12"},{"partitionParents":"[default@v2]","partitionName":"default<at:var at:name="srcpart" />ds=2008-04-09/hr=11"},{"partitionParents":"[default@v2]","partitionName":"default<at:var at:name="srcpart" />ds=2008-04-09/hr=12"}],"input_tables":[{"tablename":"default@v4","tabletype":"VIRTUAL_VIEW"},{"tablename":"default@v2","tabletype":"VIRTUAL_VIEW","tableParents":"[default@v4]"},{"tablename":"default@v1","tabletype":"VIRTUAL_VIEW","tableParents":"[default@v4]"},{"tablename":"default@src","tabletype":"MANAGED_TABLE","tableParents":"[default@v4, default@v1]"},{"tablename":"default@srcpart","tabletype":"MANAGED_TABLE","tableParents":"[default@v2]"}]}

As can be seen, src is being accessed via parents v1 and v4.

The use of AUTHORIZATION in the EXPLAIN statement shows all entities needed to be authorized to execute the query and authorization failures if exists. For example, for a query like:

EXPLAIN AUTHORIZATION
  SELECT * FROM src JOIN srcpart;

the following output is produced:

INPUTS: 
  default@srcpart
  default@src
  default@srcpart@ds=2008-04-08/hr=11
  default@srcpart@ds=2008-04-08/hr=12
  default@srcpart@ds=2008-04-09/hr=11
  default@srcpart@ds=2008-04-09/hr=12
OUTPUTS: 
  hdfs://localhost:9000/tmp/.../-mr-10000
CURRENT_USER: 
  navis
OPERATION: 
  QUERY
AUTHORIZATION_FAILURES: 
  Permission denied: Principal [name=navis, type=USER] does not have following privileges for operation QUERY [[SELECT] on Object [type=TABLE_OR_VIEW, name=default.src], [SELECT] on Object [type=TABLE_OR_VIEW, name=default.srcpart]]

With FORMATTED keyword, it will be returned in JSON format.

"OUTPUTS":["hdfs://localhost:9000/tmp/.../-mr-10000"],"INPUTS":["default@srcpart","default@src","default@srcpart@ds=2008-04-08/hr=11","default@srcpart@ds=2008-04-08/hr=12","default@srcpart@ds=2008-04-09/hr=11","default@srcpart@ds=2008-04-09/hr=12"],"OPERATION":"QUERY","CURRENT_USER":"navis","AUTHORIZATION_FAILURES":["Permission denied: Principal [name=navis, type=USER] does not have following privileges for operation QUERY [[SELECT] on Object [type=TABLE_OR_VIEW, name=default.src], [SELECT] on Object [type=TABLE_OR_VIEW, name=default.srcpart]]"]}