Execution Plan Operator

Updated on 2024-11-05 GMT+08:00

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Operator Introduction

In an SQL execution plan, each step indicates a database operator, also called an execution operator. In GaussDB(DWS), operators are the building blocks of data processing. By combining them effectively and optimizing their sequence and execution, you can significantly improve data processing efficiency.

GaussDB(DWS) operators are classified into scan operators, control operators, materialization operators, join operators, and other operators.

Scan Operators

A scan operator scans data in a table, processing one tuple at a time for the upper-layer node. It operates at the leaf node of the query plan tree and can scan tables, result sets, linked lists, and subquery results. The following table lists common scan operators.

**Table 1** Scan operators
Operator	Description	Scenario
SeqScan	Sequential scanning	It is a basic operator used to scan physical tables in sequence, not an index-assisted scan.
IndexScan	Index scanning	Indexes are created for the attributes involved in selection conditions.
IndexOnlyScan	Obtaining a tuple from an index	The index column completely overwrites the result set column.
BitmapScan(BitmapIndexScan, BitmapHeapScan)	Obtaining a tuple using a bitmap	BitmapIndexScan uses indexes for attributes to scan data and returns a bitmap. BitmapHeapScan then uses this bitmap to retrieve tuples.
TidScan	Obtaining a tuple by tuple tid	WHERE conditions(like CTID = tid or CTID IN (tid1, tid2, ...)) ; UPDATE/DELETE ... WHERE CURRENT OF cursor;
SubqueryScan	Subquery scanning	Another query plan tree (subplan) is used as the scanning object to scan tuples.
FunctionScan	Function scanning	FROM function_name
ValuesScan	Values linked list scanning	It scans the given tuple set in VALUES clauses.
ForeignScan	External table scanning	It queries external tables.
CteScan	CTE table scanning	It scans the subquery defined by the WITH clause in the SELECT query.

Join Operators

In relational algebra, a join operation is equivalent to a join operator. Take a simple example: joining two tables, t1 and t2. There are several types of joins, including inner join, left join, right join, full join, semi join, and anti join. These joins can be implemented using three methods: Nestloop, HashJoin, and MergeJoin.

**Table 2** Join operators
Operator	Description	Scenario	Implementation Feature
NestLoop	Nested loop join, which is a brute force approach. It scans the inner table for each row.	Inner Join, Left Outer Join, Semi Join, Anti Join	It is used for queries that have a smaller subset connected. In a nested loop, the foreign table drives the internal table. Each row returned by the foreign table is retrieved from the internal table to find the matched row. Therefore, the result set returned by the entire query cannot be greater than 10,000. The table with a smaller subset returned is used as the foreign table. It is recommended that indexes be created for the join fields in the internal table.
MergeJoin	A merge join on ordered input sorts both the inner and outer tables, identifies the first and last matching rows, and then joins tuples at a time. Equi-join.	Inner Join, Left Outer Join, Right Outer Join, Full Outer Join, Semi Join, Anti Join	In a merge join, data in the two joined tables is sorted by join columns. Then, data is extracted from the two tables to a sorted table for matching. A merge join requires more resources for sorting and its performance is lower than that of a hash join. However, if the source data has been pre-sorted and no more sorting is needed during the merge join, its performance excels.
(Sonic) Hash Join	Hash join: The inner and outer tables use the join column's hash value to create a hash table. Matching values are then stored in the same bucket. The two ends of an equal join must be of the same type and support hash.	Inner Join, Left Outer Join, Right Outer Join, Full Outer Join, Semi Join, Anti Join	A hash Join is used for large tables. The optimizer creates a hash table in memory using the join key and the smaller table. It then scans the larger table and uses the hash table to quickly identify matching rows. While Sonic and non-Sonic hash joins have different internal structures, this does not impact the final result set.

Materialization Operators

Materialization operators are a class of nodes that can cache tuples. During execution, many extended physical operations can be performed only after all tuples are obtained, such as aggregation function operations and sorting without indexes. Materialization operators can cache all the tuples.

**Table 3** Materialization operators
Operator	Description	Scenario
Material	Materialization	Caches the subnode result.
Sort	Sorting	ORDER BY clause, which is used for join, group, and set operations and works with Unique.
Group	Grouping	GROUP BY clause.
Agg	Executes aggregate functions.	Aggregate functions such as COUNT, SUM, AVG, MAX, and MIN. DISTINCT clause. UNION deduplication. GROUP BY clause.
WindowAgg	Window functions	WINDOW clause.
Unique	Deduplication (with sorted lower-layer data)	DISTINCT clause. UNION deduplication.
Hash	HashJoin auxiliary node	Constructs a hash table and use it together with HashJoin.
SetOp	Processing set operations	INTERSECT/INTERSECT ALL, EXCEPT/EXCEPT ALL
LockRows	Processing row-level locks	SELECT ... FOR SHARE/UPDATE

Control Operators

Control operators are a type of node that handles exceptional scenarios and executes custom workflows.

**Table 4** Control operators
Operator	Description	Scenario
Result	Performing calculation directly	1. Table scanning is not included. 2. The INSERT statement contains only one VALUES clause.
ModifyTable	INSERT/UPDATE/DELETE upper-layer node	INSERT/UPDATE/DELETE
Append	Appending	1. UNION(ALL). 2. Table inheritance.
MergeAppend	Appending (ordered input)	1. UNION(ALL). 2. Table inheritance.
RecursiveUnion	Processing the UNION subquery defined recursively in the WITH clause	WITH RECURSIVE... SELECT... statement.
BitmapAnd	Bitmap logical AND operation	BitmapScan for multi-dimensional index scanning.
BitmapOr	Bitmap logical OR operation	BitmapScan for multi-dimensional index scanning.
Limit	Processing the LIMIT clause	OFFSET ... LIMIT ...

Other Operators

Other operators include Stream and RemoteQuery. There are three types of Stream operators: Gather stream, Broadcast stream, and Redistribute stream.

Gather stream: Each source node sends its data to the target node for aggregation.
Broadcast stream: A source node sends its data to N target nodes for calculation.
Redistribute stream: Each source node calculates the hash value of its data based on the join condition, distributes the data based on the hash value, and sends the data to the corresponding target node.

**Table 5** Other Operators
Operator	Description	Scenario
Stream	Multi-node data exchange	When a distributed query plan is executed, data is exchanged between nodes.
Partition Iterator	Partition iterator	Scans partitioned tables and iteratively scans each partition.
RowToVec	Rows-to-column conversion	Hybrid row-column.
DfsScan / DfsIndexScan	HDFS table (index) scanning	HDFS table scanning.