最近准备写点Histogram和density相关的文章，先把术语给大家理一理：

cardinality (CDN)

Legend

CBQT – cost-based query transformation

JPPD – join predicate push-down

OJPPD – old-style (non-cost-based) JPPD

FPD – filter push-down

PM – predicate move-around

CVM – complex view merging

SPJ – select-project-join

SJC – set join conversion

SU – subquery unnesting

OBYE – order by elimination

OST – old style star transformation

ST – new (cbqt) star transformation

CNT – count(col) to count(*) transformation

JE – Join Elimination

JF – join factorization

SLP – select list pruning

DP – distinct placement

qb – query block

LB – leaf blocks

DK – distinct keys

LB/K – average number of leaf blocks per key

DB/K – average number of data blocks per key

CLUF – clustering factor

NDV – number of distinct values

Resp – response cost

Card – cardinality

Resc – resource cost

NL – nested loops (join)

SM – sort merge (join)

HA – hash (join)

CPUSPEED – CPU Speed

IOTFRSPEED – I/O transfer speed

IOSEEKTIM – I/O seek time

SREADTIM – average single block read time

MREADTIM – average multiblock read time

MBRC – average multiblock read count

MAXTHR – maximum I/O system throughput

SLAVETHR – average slave I/O throughput

dmeth – distribution method

1: no partitioning required

2: value partitioned

4: right is random (round-robin)

128: left is random (round-robin)

8: broadcast right and partition left

16: broadcast left and partition right

32: partition left using partitioning of right

64: partition right using partitioning of left

256: run the join in serial

0: invalid distribution method

sel – selectivity

ptn – partition

adop Automatic degree of parallelism

TABLE: Table Name

ALIAS: Table Alias

QBS: Query Block Signature

#ROWS: Number of Rows

#BLKS: Number of Blocks

ARL: Average Row Length

COR: Cardinality Original

CRD: Cardinality Rounded

CCM: Cardinality Computed

CNA: Cardinality Non Adjusted

AVGLEN: Average Column Length

NDV: Number of Distinct Values

NULLS: Number of Nulls in Column

DEN: Column Density

MIN: Minimum Column Value

MAX: Maximum Column Value

TYPE: Histogram Type

#BKTS: Histogram Buckets

UNCOMPBKTS: Histogram Uncompressed Buckets

ENDPTVALS: Histogram End Point Values

OOR: Out-of-Range Predicate

TABLE: Table Name

ALIAS: Table Alias

INDEX: Index Name

QBS: Query Block Signature

LVLS: Index Levels

#LB: Number of Leaf Blocks

#DK: Number of Distinct Keys

LB/K: Average Number of Leaf Blocks Per Key

DB/K: Average Number of Data Blocks Per Key

CLUF: Clustering Factor

INDEX_COLS: Index Column Numbers

COST: Cost of the Join

CARD: Cardinality of the Join

BC: Best Cost

LINE#: Line Number in the 10053 Trace File Where Cost Value is Located

JOIN#: Join Number in the 10053 Trace File Associated With Key

STATUS: If Permutation was Computed for all Table Joins the Status = COMPL. If Not, status = ABORT

*: In ANY Column Indicates Value Not Found in File

Freq 频率直方图

HtBal 高度平衡直方图

关于 UNCOMPBKTS和ENDPTVALS

当直方图类型为frequency histograms( Histogram: Freq)时UncompBkts  等于统计信息中采样的总行数-NULLS(Card: Original- NULLS，因为dbms_stats默认是auto_sample_size采样，所以这栏其实是采样到的原始Card-NULLS)， 而EndPtVals 等于bucket总数，或者说NDV，因为frequency histograms中 NDV=number of buckets

当直方图类型为height balanced histograms (Histogram: HtBal) UncompBkts  等于bucket的数目(其实也等于10053 trace中#Bkts的数目),而EndPtVals 等于已经被压缩的Histogram的大小，其实是等于: select count(*) from dba_tab_histograms where table_name=’YOUR_TABLE_NAME’ and column_name=’YOUR_COLUMN_NAME’的实际总和。  通过这2个值对比，可以了解到popular值的多少以及数据的倾斜度， 是有多个大量重复的值(popular value)还是仅有一个巨大的重复值。