Hi

This command is basically used when a large processing of 
data has occurred. If a large amount of deletions any 
modification or Bulk Copy into the tables has occurred, it 
has to update the indexes to take these changes into 
account. UPDATE_STATISTICS updates the indexes on these 
tables accordingly.

The UPDATE STATISTICS statement defines the storage 
requirements of tables and indexes as well as the value 
distribution of columns, and stores this information in the 
database catalog.

Syntax
<update statistics statement> ::= 
  UPDATE STAT[ISTICS] COLUMN <table name>.<column name> 
[ESTIMATE [<sample definition>]]
| UPDATE STAT[ISTICS] COLUMN (<column name>,...) FOR <table 
name> [ESTIMATE [<sample definition>]]
| UPDATE STAT[ISTICS] COLUMN (*) FOR <table name> [ESTIMATE 
[<sample definition>]]
| UPDATE STAT[ISTICS] <table name> [ESTIMATE [<sample 
definition>]]
| UPDATE STAT[ISTICS] [<owner>.][<identifier>]* [ESTIMATE 
[<sample definition>]]