lookupscan.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.*;

/**
 * The scan class corresponding to the lookup relational
 * algebra operator.
 * @author Edward Sciore
 */
public class LookupScan implements Scan {
	private RidScan rs;
	private TableScan ts;

	/**
	 * Creates a lookup scan for the specified rid-scan
	 * and table scan.
	 * Note that the table scan is <i>not</i> used for scanning.
	 * Instead, records from the table scan will be read
	 * by looking up their RIDs.
	 * @param rs the rid-scan
	 * @param ts the table scan
	 */
	public LookupScan(RidScan rs, TableScan ts) {
		this.rs = rs;
		this.ts = ts;
		beforeFirst();
	}

	/**
	 * Positions the scan before the first record, which
	 * in this case is before the first rid-scan record.
	 * @see simpledb.query.Scan#beforeFirst()
	 */
	public void beforeFirst() {
		rs.beforeFirst();
	}

	/**
	 * Moves to the next rid-scan record.
	 * If there is such a record, then get the dataRid
	 * of that record and read in the corresponding 
	 * record from the table scan.
	 * @see simpledb.query.Scan#next()
	 */
	public boolean next() {
		boolean ok = rs.next();
		if (ok) {
			RID rid = rs.getDataRid();
			ts.moveToRid(rid);
		}
		return ok;
	}

	/**
	 * Closes the scan by closing the rid-scan and the table scan.
	 * @see simpledb.query.Scan#close()
	 */
	public void close() {
		rs.close();
		ts.close();
	}

	/**
	 * Returns the Constant value of the specified field.
	 * This value will come either from the current
	 * rid-scan record or from the looked-up 
	 * table scan record.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public Constant getVal(String fldname) {
		if (ts.hasField(fldname))
			return ts.getVal(fldname);
		else
			return rs.getVal(fldname);
	}

	/**
	 * Returns the integer value of the specified field.
	 * This value will come either from the current
	 * rid-scan record or from the looked-up 
	 * table scan record.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public int getInt(String fldname) {
		if (ts.hasField(fldname))
			return ts.getInt(fldname);
		else
			return rs.getInt(fldname);
	}

	/**
	 * Returns the string value of the specified field.
	 * This value will come either from the current
	 * rid-scan record or from the looked-up 
	 * table scan record.
	 * @see simpledb.query.Scan#getVal(java.lang.String)
	 */
	public String getString(String fldname) {
		if (ts.hasField(fldname))
			return ts.getString(fldname);
		else
			return rs.getString(fldname);
	}

	/**
	 * Returns true if the specified field is in
	 * either the rid-scan or the table scan.
	 * @see simpledb.query.Scan#hasField(java.lang.String)
	 */
	public boolean hasField(String fldname) {
		return ts.hasField(fldname) || rs.hasField(fldname);
	}
}