mysqlfetchsize原理_MySQLJDBCFetchSize解析
⾸先是看Java JDBC的API查看tFetchSize:
tFetchSize(int rows)
Givesthe JDBC driver a hint as to the number of rows that should be fetched from thedataba when more rows are needed for this ResultSetobject.
查看的结果给出这⾥的rows只是⼀个hint，那么对于MySQL JDBC来说是如何实现的呢？
There is experimental support for fetching rows in batches
When using Connector/J 5.0.1 along with more recent builds of the MySQL rver, you can add "uCursorFetch=true" to your JDBC url parameters, and the driver will fetch rows in batches of size tFetchSize() as defined in the JDBC API.
One could also argue that the behavior _does_ follow the JDBC API, quoting from the APIDOCS for
Statement.tFetchSize():
理字笔顺
"Gives the JDBC driver a hint as to the number of rows that should be fetched from the databa when more rows are needed. The number of rows specified affects only result ts created using this statement. If the value specified is zero, then the hint is ignored. The default value is zero."
午睡睡不着
It‘s only a _hint_. Driver vendors are allowed to ignore hints. The very reason that the wording is there is becau there are quite a few vendors who can not adhere to this "contract" in all situations.
uCursorFetch
If connected to MySQL > 5.0.2, and tFetchSize() > 0 on a statement, should that statement u cursor-bad fetching to retrieve rows?战略规划方案
fal
⼜说是experiment，⼜说是hint可能被ignore，到底⽀持还不不⽀持呢？并且官⽅⽂档给出的这个参数没说到底是不是会忽略掉fetchSize.按照故事的尿性来说我该看MySQL JDBC的源码了:
辅导学习1.⾸先判断是否可以进⾏cursor read
if (tion.versionMeetsMinimum(5, 0, 2)
&& UCursorFetch()
&& isBinaryEncoded
&& callingStatement != null
&& FetchSize() != 0
&& ResultSetType() == ResultSet.TYPE_FORWARD_ONLY)
若OK，则RowData rows = new RowDataCursor
2.如果不满⾜，则判断是否可以进⾏Streaming Read：
sultSetType== java.sql.ResultSet.TYPE_FORWARD_ONLY)
&& (sultSetConcurrency == java.sql.ResultSet.CONCUR_READ_ONLY) && (this.fetchSize == Integer.MIN_VALUE)
如果不满⾜则rowData = readSingleRowSet
3.否则rowData = new RowDataDynamic
这⾥主要关注RowDataCursor的next实现⽅式：next会去调⽤fetchMoreRows，
3.1 如果fetchsize==Integer.MIN_VALUE，则另fetchsize=1，类似stream 模式
3.2 否则回调fetchRowsViaCursor：
this.sharedSendPacket.writeByte((byte) MysqlDefs_FETCH);
this.sharedSendPacket.writeLong(statementId);
this.sharedSendPacket.writeLong(fetchSize);
ndCommand(MysqlDefs_FETCH, null, this.sharedSendPacket, true,
null);
while ((row = nextRow(columnTypes, columnTypes.length, true,
ResultSet.CONCUR_READ_ONLY, fal, uBufferRowExplicit, fal, null)) != null) { fetchedRows.add(row);
}
当执⾏ndCommand后，看MySQL源码是如何处理的：
317 void Materialized_cursor::fetch(ulong num_rows)
318 {
319 THD *thd= table->in_u;
320
321 int res=0;
322 result->begin_datat();
323 for(fetch_limit+= num_rows; fetch_count < fetch_limit; fetch_count++)
324 {
325 if((res= table->file->rnd_next(table->record[0])))
326 break;
327 /* Send data only if the read was successful. */
328 /*纺织业
329 If network write failed (i.e. due to a clod socked),
330 the error has already been t. Just return.
331 */
332 if(result->nd_data(item_list))
廉洁自律条例333 return;
334 }
335
336 switch(res) {
337 ca0:
338 thd->rver_status|= SERVER_STATUS_CURSOR_EXISTS;
339 result->nd_eof();
340 break;
341 ca HA_ERR_END_OF_FILE:
342 thd->rver_status|= SERVER_STATUS_LAST_ROW_SENT; 343 result->nd_eof();
344 clo();
345 break;
346 default:
347 table->file->print_error(res,MYF(0));
胸闷气短快速解决办法
348 clo();
349 break;
350 }
351 }圣诞节是什么节
分析到这⾥基本上可以确定MySQL也是⽀持batch fetch的.
