SELECT

The SELECT command returns a set of records from one or more tables.

query:
  |   WITH withItem [ , withItem ]* query
  |   {
          select
      }
      [ ORDER BY orderItem [, orderItem ]* ]
      [ LIMIT [ start, ] { count | ALL } ]
      [ OFFSET start { ROW | ROWS } ]

withItem:
      name
      [ '(' column [, column ]* ')' ]
      AS '(' query ')'

orderItem:
      expression [ ASC | DESC ] [ NULLS FIRST | NULLS LAST ]

select:
      SELECT [ DISTINCT ] [/*+ hints */]
          { * | projectItem [, projectItem ]* }    
      FROM tableExpression
      [ WHERE booleanExpression ]
      [ GROUP BY { groupItem [, groupItem ]* } ]
      [ HAVING booleanExpression ]
      [ WINDOW window_name AS ( window_definition ) [, ...] ]

projectItem:
      expression [ [ AS ] columnAlias ]
  |   tableAlias . *

tableExpression:
      tableReference [, tableReference ]*
  |   tableExpression [ ( LEFT ) [ OUTER ] ] JOIN tableExpression [ joinCondition ]

joinCondition:
      ON booleanExpression
  |   USING '(' column [, column ]* ')'

tableReference:
      tablePrimary
      [ [ AS ] alias ]

tablePrimary:
      [ catalogName . ] tableName
  |   '(' query ')'

groupItem:
      expression
  |   '(' expression [, expression ]* ')'

For more information, see SELECT.

ORDER BY

• Sort order defaults to ascending (ASC).

• Sorts null values after non-null values by default in an ascending sort, before non-null values in a descending sort. For any query, you can use NULLS FIRST to sort null values to the top of the results or NULLS LAST to sort null values to the bottom of the results.

• Allows you to use a positional reference to choose the sort column. For example, the command SELECT colA,colB FROM table1 ORDER BY 2 sorts the results on colB because it is in position 2.

Query Hints

HEAVY.AI provides various query hints for controlling the behavior of the query execution engine.

Syntax

SELECT /*+ hint */ FROM ...;

SELECT hints must appear first, immediately after the SELECT statement; otherwise, the query fails.

By default, a hint is applied to the query step in which it is defined. If you have multiple SELECT clauses and define a query hint in one of those clauses, the hint is applied only to the specific query step; the rest of the query steps are unaffected. For example, applying the /* cpu_mode */ hint affects only the SELECT clause in which it exists.

You can define a hint to apply to all query steps by prepending g_ to the query hint. For example, if you define /*+ g_cpu_mode */, CPU execution is applied to all query steps.

HEAVY.AI supports the following query hints.

Marker

The marker hint type represents a Boolean flag.

Hint	Details	Example
allow_loop_join	Enable loop joins.	SELECT /+* allow_loop_join */ ...
cpu_mode	Force CPU execution mode.	SELECT /*+ cpu_mode */ ...
columnar_output	Enable columnar output for the input query.	SELECT /+* columnar_output */ ...
disable_loop_join	Disable loop joins.	SELECT /+* disable_loop_join */ ...
dynamic_watchdog	Enable dynamic watchdog.	SELECT /+* dynamic_watchdog */ ...
dynamic_watchdog_off	Disable dynamic watchdog.	SELECT /+* dynamic_watchdog_off */ ...
force_baseline_hash_join	Use the baseline hash join scheme by skipping the perfect hash join scheme, which is used by default.	SELECT /+* force_baseline_hash_join */ ...
force_one_to_many_hash_join	Deploy a one-to-many hash join by skipping one-to-one hash join, which is used by default.	SELECT /+* force_one_to_many_hash_join */ ...
keep_result	Add result set of the input query to the result set cache.	SELECT /+* keep_result */ ...
keep_table_function_result	Add result set of the table function query to the result set cache.	SELECT /+* keep_table_function_result */ ...
overlaps_allow_gpu_build	Use GPU (if available) to build an overlaps join hash table. (CPU is used by default.)	SELECT /+* overlaps_allow_gpu_build */ ...
overlaps_no_cache	Skip adding an overlaps join hash table to the hash table cache.	SELECT /+* overlaps_no_cache */ ...
rowwise_output	Enable row-wise output for the input query.	SELECT /+* rowwise_output */ ...
watchdog	Enable watchdog.	SELECT /+* watchdog */ ...
watchdog_off	Disable watchdog.	SELECT /+* watchdog_off */ ...

Key-Value pair

The key-value pair type is a hint name and its value.

Hint	Details	Example
aggregate_tree_fanout	Defines a fan out of a tree used to compute window aggregation over frame. Depending on the frame size, the tree fanout affects the performance of aggregation and the tree construction for each window function with a frame clause. Value type: INT Range: 0-1024	SELECT /+* aggregate_tree_fanout(32) */ SUM(y) OVER (ORDER BY x ROWS BETWEEN ...) ...
loop_join_inner_table_max_num_rows	Set the maximum number of rows available for a loop join. Value type: INT Range: 0 < x	Set the maximum number of rows to 100: SELECT /+* loop_join_inner_table_max_num_rows(100) */ ...
max_join_hash_table_size	Set the maximum size of the hash table. Value type: INT Range: 0 < x	Set the maximum size of the join hash table to 100: SELECT /+* max_join_hash_table_size(100) */ ...
overlaps_bucket_threshold	Set the overlaps bucket threshold. Value type: DOUBLE Range: 0-90	Set the overlaps threshold to 10: SELECT /*+ overlaps_bucket_threshold(10.0) */ ...
overlaps_max_size	Set the maximum overlaps size. Value type: INTEGER Range: >=0	Set the maximum overlap to 10: SELECT /*+ overlaps_max_size(10.0) */ ...
overlaps_keys_per_bin	Set the number of overlaps keys per bin. Value type: DOUBLE Range: 0.0 < x < double::max	SELECT /+* overlaps_keys_per_bin(0.1) */ ...
query_time_limit	Set the maximum time for the query to run. Value type: INTEGER Range: >=0	SELECT /+* query_time_limit(1000) */ ...

Cross-Database Queries

In Release 6.4 and higher, you can run SELECT queries across tables in different databases on the same HEAVY.AI cluster without having to first connect to those databases. This enables more efficient storage and memory utilization by eliminating the need for table duplication across databases, and simplifies access to shared data and tables.

To execute queries against another database, you must have ACCESS privilege on that database, as well as SELECT privilege.

Example

Execute a join query involving a table in the current database and another table in the my_other_db database:

SELECT name, saleamt, saledate FROM my_other_db.customers AS c, sales AS s 
  WHERE c.id = s.customerid;