|SQL Language Reference / Statements|
A query with an optional ORDER BY CLAUSE and an optional FOR UPDATE clause.
A SELECT statement consists of a query with an optional ORDER BY CLAUSE and an optional FOR UPDATE clause. The SELECT statement is so named because the typical first word of the query construct is SELECT. (Query includes the VALUES expression as well as SELECT expressions).
The ORDER BY CLAUSE guarantees the ordering of the ResultSet. The FOR UPDATE clause acquires a lock on the selected rows and makes the result set's cursor updatable.
A SELECT statement returns a ResultSet. A cursor is a pointer to a specific row in ResultSet. In Java applications, all ResultSets have an underlying associated SQL cursor, often referred to as the result set's cursor. The cursor can be updatable, that is, you can update or delete rows as you step through the ResultSet if the SELECT statement that generated it and its underlying query meet cursor updatability requirements, as detailed below. In GemFire XD, the FOR UPDATE clause is required in order to obtain an updatable cursors, or to limit the columns that can be updated. GemFire XD also requires that you use a peer client connection in order to obtain an updateable result set. See Updatable Result Sets Limitations for more information about result set limitations.
-- Query only the in-memory data, avoiding a costly table scan. select count(*) from hdfstable; -- Query the in-memory, operational data set. select count(*) from hdfstable --GEMFIREXD-PROPERTIES queryHDFS=true;
The SELECT depends on all the tables and views named in the query and the conglomerates (units of storage such as heaps and indexes) chosen for access paths on those tables.
CREATE INDEX does not invalidate a prepared SELECT statement.
A DROP INDEX statement invalidates a prepared SELECT statement if the index is an access path in the statement. If the SELECT includes views, it also depends on the dictionary objects on which the view itself depends (see CREATE VIEW statement).
Any prepared UPDATE WHERE CURRENT or DELETE WHERE CURRENT statement against a cursor of a SELECT depends on the SELECT. Removing a SELECT through a java.sql.Statement.close request invalidates the UPDATE WHERE CURRENT or DELETE WHERE CURRENT.
The SELECT depends on all synonyms used in the query. Dropping a synonym invalidates a prepared SELECT statement if the statement uses the synonym.
-- lists the customer ID and expression -- LOANLIMIT-AVAILLOAN as REMAINING_LIMIT and -- orders by the new name REMAINING_LIMIT SELECT CID, LOANLIMIT-AVAILLOAN AS REMAINING_LIMIT FROM TRADE.NETWORTH ORDER BY REMAINING_LIMIT -- creating an updatable cursor with a FOR UPDATE clause -- to update the SINCE date -- column in the CUSTOMERS table SELECT SINCE FROM TRADE.CUSTOMERS FOR UPDATE OF SINCE