参数optimizer_dynamic_sampling用于控制动态采样的级别，10g中默认为2，9.2.0为1，以下版本为0，它的最高级别为10。
level级别和描述如下：

Level 0: Perform no dynamic sampling at all.------不进行动态采样

Level 1: Sample tables that appear in join or sub-query conditions that have no indexes and have more blocks than 32 (the default for dynamic sampling)------被采样的表必须满足以下条件:被采样的表必须出现在join或者子查询中，并且没有索引，而且blocks必须大于32(默认采样数)

Level 2: Dynamically sample all unanalyzed tables that have more than 32 blocks.------采样block数大于32并且没有被分析的表

Level 3: Dynamic samples tables using a single column that applies selectivity to the table being sampled in addition to the level 2 rules.------应用level2的规则，并且在单列上应用选择性规则检查

Level 4: Dynamic samples tables using 2 or more columns that applies selectivity to the table being sampled in addition to the level 3 rules.------应用level3的规则，并且在2列，或者多余2列上应用选择性规则检查

Level 5: Dynamic samples up to 64 blocks on tables using level 4 rules. The table of course has to be larger than 64 blocks in size (2 times the default sampling size).------应用level4的规则，并且被采样的表的block数必须大于64，同时抽取64个block采样

Level 6: Dynamic samples up to 128 blocks on tables using level 4 rules. The table, of course, has to be larger than 128 blocks in size (4 times the default sampling size).------应用level4的规则，并且被采样的表的block数必须大于128，同时抽取128个block采样

Level 7: Dynamic samples up to 256 blocks on tables using level 4 rules. The table, of course, has to be larger than 256 blocks in size (8 times the default sampling size).-------应用level4的规则，并且被采样的表的block数必须大于256，同时抽取256个block采样

Level 8: Dynamic samples up to 1024 blocks on tables using level 4 rules. The table, of course, has to be larger than 1024 blocks in size (32 times the default sampling size).-------应用level4的规则，并且被采样的表的block数必须大于1024，同时抽取1024个block采样

Level 9: Dynamic samples up to 4096 blocks on tables using level 4 rules. The table has to be larger than 4096 blocks in size (128 times the default sampling size).-------应用level4的规则，并且被采样的表的block数必须大于4096，同时抽取4096个block采样

Level 10: Dynamic sampling of all blocks on tables using level 4 rules-------应用level4的规则，并且抽取所有的blocks进行采样