indexselectscan.java

用java语言简单实现数据库的初步功能

package simpledb.index.query;

import simpledb.record.RID;
import simpledb.query.*;
import simpledb.index.Index;

/**
 * The scan class corresponding to the select relational
 * algebra operator.
 * @author Edward Sciore
 */
public class IndexSelectScan implements RidScan {
	private Index idx;
	private Constant val;

	/**
	 * Creates an index select scan for the specified
	 * index and selection constant.
	 * @param idx the index
	 * @param val the selection constant
	 */
	public IndexSelectScan(Index idx, Constant val) {
		this.idx = idx;
		this.val = val;
		beforeFirst();
	}

	/**
	 * Positions the scan before the first record,
	 * which in this case means positioning the index
	 * before the first instance of the selection constant.
	 * @see simpledb.query.Scan#beforeFirst()
	 */
	public void beforeFirst() {
		idx.beforeFirst(val);
	}

	/**
	 * Moves to the next record, which in this case means
	 * moving the index to the next record satisfying the
	 * selection constant, and returning false if there are
	 * no more such index records.
	 * @see simpledb.query.Scan#next()
	 */
	public boolean next() {
		return idx.next();
	}

	/**
	 * Closes the scan by closing the index.
	 * @see simpledb.query.Scan#close()
	 */
	public void close() {
		idx.close();
	}

	/**
	 * Throws an exception, because the schema contains
	 * no fields.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public Constant getVal(String fldname) {
		throw new RuntimeException("field " + fldname + " not found.");
	}

	/**
	 * Throws an exception, because the schema contains
	 * no fields.
	 * @see simpledb.query.Scan#getInt(java.lang.String)
	 */
	public int getInt(String fldname) {
		throw new RuntimeException("field " + fldname + " not found.");
	}

	/**
	 * Throws an exception, because the schema contains
	 * no fields.
	 * @see simpledb.query.Scan#getString(java.lang.String)
	 */
	public String getString(String fldname) {
		throw new RuntimeException("field " + fldname + " not found.");
	}

	/**
	 * Always return false, because the schema has no fields.
	 * @see simpledb.query.Scan#hasField(java.lang.String)
	 */
	public boolean hasField(String fldname) {
		return false;
	}

	/**
	 * Returns the dataRid value of the current record,
	 * which is obtained from the current index record.
	 * @see simpledb.index.query.RidScan#getDataRid()
	 */
	public RID getDataRid() {
		return idx.getDataRid();
	}
}