indexselectplan.java

用java语言简单实现数据库的初步功能

package simpledb.index.query;

import simpledb.tx.Transaction;
import simpledb.record.Schema;
import simpledb.query.*;
import simpledb.index.Index;
import simpledb.index.metadata.IndexInfo;

/** The Plan class corresponding to the <i>indexselect</i>
 * relational algebra operator.
 * @author Edward Sciore
 */
public class IndexSelectPlan implements Plan {
	private IndexInfo ii;
	private Constant val;
	
	/**
	 * Creates a new indexselect node in the query tree
	 * for the specified index and selection constant.
	 * @param ii information about the index
	 * @param val the selection constant
	 * @param tx the calling transaction 
	 */
	public IndexSelectPlan(IndexInfo ii, Constant val, Transaction tx) {
		this.ii = ii;
		this.val = val;
	}

	/** 
	 * Creates a new indexselect scan for this query
	 * @see simpledb.query.Plan#open()
	 */
	public Scan open() {
		Index idx = ii.open();
		return new IndexSelectScan(idx, val);
	}

	/**
	 * Estimates the number of block accesses to compute the 
	 * index selection, which is the same as the 
	 * index traversal cost.
	 * @see simpledb.query.Plan#blocksAccessed()
	 */
	public int blocksAccessed() {
		return ii.blocksAccessed();
	}

	/**
	 * Estimates the number of output records in the index selection,
	 * which is the same as the number of search key values
	 * for the index.
	 * @see simpledb.query.Plan#recordsOutput()
	 */
	public int recordsOutput() {
		return ii.recordsOutput();
	}

	/** 
	 * This method should never be called,
	 * because there are no interesting fields in the index.
	 * @see simpledb.query.Plan#distinctValues(java.lang.String)
	 */
	public int distinctValues(String fname) {
		throw new RuntimeException("field " + fname + " not found.");
	}

	/**
	 * Returns an empty schema, because there are no
	 * interesting fields in the index.
	 * @see simpledb.query.Plan#schema()
	 */
	public Schema schema() {
		return new Schema();  // i.e. return an empty schema
	}
}