JDBC读取数据优化-fetch size
最近由于业务上的需求，一张旧表结构中的数据，需要提取出来，根据规则，导入一张新表结构中，开发同学写了一个工具，用于实现新旧结构的transformation，
实现逻辑简单，就是使用jdbc从A表读出数据，做了一些处理，再存入新表B中，发现读取旧表的操作，非常缓慢，无法满足要求。
读取数据的示例代码，
conn = getConnection();
long start = System.currentTimeMillis();
ps = conn.prepareStatement(sql);
rs = ps.executeQuery();
long mid_end = System.currentTimeMillis();
while (rs.next()) {
list.add(rs.getString(1));
}
long end = System.currentTimeMillis();
rs.close();
System.out.println("Interval1=" + (mid_end - start));
System.out.println("Interval2=" + (end - mid_end));
SQL语句读取10000条记录，其中，
Interval1=160ms
Interval2=29252ms
执行executeQuery()这个SQL检索的时间为160毫秒。
执行10000次rs.next以及rs.getString(1)的用时约为30秒，平均1条记录3毫秒。
如何才能提高读取的效率？
上面读取10000条记录，每一次rs.next时间只有3毫秒，但是由于需要10000次，所以才需要30秒，我们可以猜测，是否有可能每一次rs.next的执行，均需要和数据库交互，因为如果仅是字符串操作，不应该是这个数量级。
看一下官方文档的描述，《Database JDBC Developer's Guide》有一节介绍了Fetch Size，
By default, when Oracle JDBC runs a query, it retrieves a result set of 10 rows at a time from the database cursor. This is the default Oracle row fetch size value. You can change the number of rows retrieved with each trip to the database cursor by changing the row fetch size value.
Standard JDBC also enables you to specify the number of rows fetched with each database round-trip for a query, and this number is referred to as the fetch size. In Oracle JDBC, the row-prefetch value is used as the default fetch size in a statement object. Setting the fetch size overrides the row-prefetch setting and affects subsequent queries run through that statement object.
Fetch size is also used in a result set. When the statement object run a query, the fetch size of the statement object is passed to the result set object produced by the query. However, you can also set the fetch size in the result set object to override the statement fetch size that was passed to it.
Changes made to the fetch size of a statement object after a result set is produced will have no affect on that result set.
JDBC默认每执行一次检索，会从游标中提取10行记录，10就是默认的row fetch size值，通过设置row fetch size，可以改变每次和数据库交互，提取出来的记录行总数。需要注意的是，需要在获得检索结果集之前，设置fetch size，否则就是无效。
可以使用如下方法设置，
Setting the Fetch Size
The following methods are available in all Statement, PreparedStatement, CallableStatement, and ResultSet objects for setting and getting the fetch size:
void setFetchSize(int rows) throws SQLException
int getFetchSize() throws SQLException
简单来讲，Fetch相当于读缓存，默认Fetch Size值是10，读取10000条记录，一次数据库交互，即rs.next的操作，ResultSet会一次性从数据库服务器，得到10条记录，下次执行rs.next，就直接使用内存读取，不用和数据库交互了，但总计需要有1000次交互，如果使用setFetchSize设置Fetch Size为10000，则只需要一次数据库交互，本地缓存10000条记录，每次执行rs.next，只是内存操作，不会有数据库网络消耗，效率就会高些。但需要注意的是，Fetch Size值越高则占用内存越高，要避免出现OOM错误。
方案1：
rs = ps.executeQuery();
rs.setFetchSize(10000);
即在执行ps.executeQuery()之后，对rs设置值10000，统计如下，
执行executeQuery()这个SQL检索的时间为174毫秒。
执行10000次rs.next以及rs.getString(1)的用时约为190毫秒。
相比之前执行10000次rs.next，用了30秒，提高了将近150倍。
方案2：
ps = conn.prepareStatement(sql);
ps.setFetchSize(10000);
即在执行conn.prepareStatement(sql)之后，执行ps.executeQuery()之前，对rs设置值为10000范围，统计如下，
执行executeQuery()这个SQL检索的时间为267毫秒。
执行10000次rs.next以及rs.getString(1)的用时约为87毫秒。
相比方案2，总用时几乎一致，但SQL执行和rs.next遍历的用时，有些区别。
针对方案1，
After you have run the query, you can call
setFetchSizeon the result set object to override the statement object fetch size that was passed to it. This will affect any subsequent trips to the database to get more rows for the original query, as well as affecting any later refetching of rows.
执行查询之后，对结果集设置setFetchSize，会影响任何接下来的数据库交互过程获得更多的记录行数，以及之后的fetch提取。
针对方案2，
To set the fetch size for a query, call
setFetchSizeon the statement object prior to running the query. If you set the fetch size to N, then N rows are fetched with each trip to the database.
执行查询之前，设置setFetchSize，表示每次和数据库交互，得到记录行数。
综上所述，建议执行SQL之前，设置此值，效率提升最高。
对于PrepareStatement、ResultSet和Statement，均有这一个方法，有一点出入的，就是默认值设置(0)，从代码中使用getFetchSize()，得到的值均为10，不知道是我理解错了，还是有其他含义？欢迎各位指教。
PrepareStatement
setFetchSize
void setFetchSize(int rows)
throws SQLExceptionGives the JDBC driver a hint as to the number of rows that should be fetched from the database when more rows are needed for
ResultSetobjects generated by thisStatement. If the value specified is zero, then the hint is ignored. The default value is zero.
Parameters:
rows - the number of rows to fetch
Throws:
SQLException - if a database access error occurs, this method is called on a closed Statement or the condition rows >= 0 is not satisfied.
Since:
1.2
See Also:
getFetchSize()
ResultSet
setFetchSize
void setFetchSize(int rows)
throws SQLExceptionGives the JDBC driver a hint as to the number of rows that should be fetched from the database when more rows are needed for this
ResultSetobject. If the fetch size specified is zero, the JDBC driver ignores the value and is free to make its own best guess as to what the fetch size should be. The default value is set by theStatementobject that created the result set. The fetch size may be changed at any time.
Parameters:
rows - the number of rows to fetch
Throws:
SQLException - if a database access error occurs; this method is called on a closed result set or the condition rows >= 0 is not satisfied
Since:
1.2
See Also:
getFetchSize()
Statement
setFetchSize
void setFetchSize(int rows)
throws SQLExceptionGives the JDBC driver a hint as to the number of rows that should be fetched from the database when more rows are needed for
ResultSetobjects generated by thisStatement. If the value specified is zero, then the hint is ignored. The default value is zero.
Parameters:
rows - the number of rows to fetch
Throws:
SQLException - if a database access error occurs, this method is called on a closed Statement or the condition rows >= 0 is not satisfied.
Since:
1.2
See Also:
getFetchSize()
总结：
1. Fetch相当于读缓存，如果使用setFetchSize设置Fetch Size为10000，本地缓存10000条记录，每次执行rs.next，只是内存操作，不会有数据库网络消耗，效率就会高些。但需要注意的是，Fetch Size值越高则占用内存越高，要避免出现OOM错误。
2. 建议执行SQL语句之前设置，即ps.executeQuery();之前使用setFetchSize()函数设置。
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