使用Mycat感触最深的是：有了Mycat后端都是Mysql数据库了，对外屏蔽了数据库底层，你压根不知道后端使用的啥数据库，甚至是分页你按照Mysql的Limit语法传递参数就没有问题，因为后端的PageSQLUtil会自动包装转换.........
PageSQLUtil类目前兼容ORACLE、SQL_SERVER、DB2、POSTGRESQL、MYSQL分页。
public class PageSQLUtil
{
public static String convertLimitToNativePageSql(String dbType, String sql,
int offset, int count)
{
if (JdbcConstants.ORACLE.equalsIgnoreCase(dbType))
{
OracleStatementParser oracleParser = new OracleStatementParser(sql);
SQLSelectStatement oracleStmt = (SQLSelectStatement) oracleParser.parseStatement();
return PagerUtils.limit(oracleStmt.getSelect(), JdbcConstants.ORACLE, offset, count);
} else if (JdbcConstants.SQL_SERVER.equalsIgnoreCase(dbType))
{
SQLServerStatementParser oracleParser = new SQLServerStatementParser(sql);
SQLSelectStatement sqlserverStmt = (SQLSelectStatement) oracleParser.parseStatement();
SQLSelect select = sqlserverStmt.getSelect();
SQLOrderBy orderBy= select.getOrderBy() ;
if(orderBy==null)
{
SQLSelectQuery sqlSelectQuery= select.getQuery();
if(sqlSelectQuery instanceof SQLServerSelectQueryBlock)
{
SQLServerSelectQueryBlock sqlServerSelectQueryBlock= (SQLServerSelectQueryBlock) sqlSelectQuery;
SQLTableSource from= sqlServerSelectQueryBlock.getFrom();
if("limit".equalsIgnoreCase(from.getAlias()))
{
from.setAlias(null);
}
}
SQLOrderBy newOrderBy=new SQLOrderBy(new SQLIdentifierExpr("(select 0)"));
select.setOrderBy(newOrderBy);
}
return PagerUtils.limit(select, JdbcConstants.SQL_SERVER, offset, count) ;
}
else if (JdbcConstants.DB2.equalsIgnoreCase(dbType))
{
DB2StatementParser db2Parser = new DB2StatementParser(sql);
SQLSelectStatement db2Stmt = (SQLSelectStatement) db2Parser.parseStatement();
//注意此处没有调用PagerUtils类中的方法，也许PagerUtils中的limitDB2这个方法有bug
//因为PagerUtils 来自druid-1.0.26包中的com.alibaba.druid.sql.PagerUtils
return limitDB2(db2Stmt.getSelect(), JdbcConstants.DB2, offset, count);
} else if (JdbcConstants.POSTGRESQL.equalsIgnoreCase(dbType))
{
PGSQLStatementParser pgParser = new PGSQLStatementParser(sql);
SQLSelectStatement pgStmt = (SQLSelectStatement) pgParser.parseStatement();
SQLSelect select = pgStmt.getSelect();
SQLSelectQuery query= select.getQuery();
if(query instanceof PGSelectQueryBlock)
{
PGSelectQueryBlock pgSelectQueryBlock= (PGSelectQueryBlock) query;
pgSelectQueryBlock.setOffset(null);
pgSelectQueryBlock.setLimit(null);
}
return PagerUtils.limit(select, JdbcConstants.POSTGRESQL, offset, count);
} else if (JdbcConstants.MYSQL.equalsIgnoreCase(dbType))
{
MySqlStatementParser pgParser = new MySqlStatementParser(sql);
SQLSelectStatement pgStmt = (SQLSelectStatement) pgParser.parseStatement();
SQLSelect select = pgStmt.getSelect();
SQLSelectQuery query= select.getQuery();
if(query instanceof MySqlSelectQueryBlock)
{
MySqlSelectQueryBlock pgSelectQueryBlock= (MySqlSelectQueryBlock) query;
pgSelectQueryBlock.setLimit(null);
}
return PagerUtils.limit(select, JdbcConstants.MYSQL, offset, count);
}
return sql;
}
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附件：实验代码
EXPLAIN select * from dept limit 4,3
truncate table dept
INSERT into dept(uuid,id,address) values(1,1,'北京');
INSERT into dept(uuid,id,address) values(2,2,'上海');
INSERT into dept(uuid,id,address) values(3,3,'深圳');
INSERT into dept(uuid,id,address) values(4,4,'广州');
INSERT into dept(uuid,id,address) values(5,5,'天津');
INSERT into dept(uuid,id,address) values(6,6,'杭州');
INSERT into dept(uuid,id,address) values(7,7,'南京');
INSERT into dept(uuid,id,address) values(8,8,'济南');
INSERT into dept(uuid,id,address) values(9,9,'重庆');
INSERT into dept(uuid,id,address) values(10,10,'青岛');
INSERT into dept(uuid,id,address) values(11,11,'大连');
INSERT into dept(uuid,id,address) values(12,12,'宁波');
INSERT into dept(uuid,id,address) values(13,13,'厦门');
INSERT into dept(uuid,id,address) values(14,14,'成都');
INSERT into dept(uuid,id,address) values(15,15,'武汉');
INSERT into dept(uuid,id,address) values(16,16,'哈尔滨');
INSERT into dept(uuid,id,address) values(17,17,'沈阳');
INSERT into dept(uuid,id,address) values(18,18,'西安');
INSERT into dept(uuid,id,address) values(19,19,'长春');
INSERT into dept(uuid,id,address) values(20,20,'长沙');
INSERT into dept(uuid,id,address) values(21,21,'福州');
INSERT into dept(uuid,id,address) values(22,22,'郑州');
INSERT into dept(uuid,id,address) values(23,23,'石家庄');
INSERT into dept(uuid,id,address) values(24,24,'苏州');
INSERT into dept(uuid,id,address) values(25,25,'佛山');
INSERT into dept(uuid,id,address) values(26,26,'东莞');
INSERT into dept(uuid,id,address) values(27,27,'无锡');
INSERT into dept(uuid,id,address) values(28,28,'烟台');
INSERT into dept(uuid,id,address) values(29,29,'太原');