Question about an explain plan.

Giganews Newsgroups
Subject: Question about an explain plan.
Posted by:  dimitr…@cs.umb.edu
Date: 21 Oct 2005

Hi,

I got the following explain plan for a query:

Order    Execution Step    Object Name    Object Owner    Est Cost    Est Row
Count    Est Byte Count
7        [-] Select Statement [CHOOSE]            12    1    102
6            [-] Nested Loops            12    1    102
3                [-] Sort (Unique)
2                    [-] Table Access (By Index Rowid)
[ANALYZED]    EMAILLATEST    XXXXXXXX    5    1    20
1                        [ ] Index (Range Scan)
[ANALYZED]    XPKEMAILLATEST    XXXXXXXX    4    1
5                [-] Table Access (By Index Rowid)
[ANALYZED]    EMAIL    XXXXXXXX    3    1    82
4                    [ ] Index (Unique Scan)
[ANALYZED]    XPKEMAIL    XXXXXXXX    2    28,940,028

One thing I cannot understand in this plan is that the last step,
which is a unique scan of an index, contains a very large number as
the estimated row count (28,940,028).  Do you know what this number
would mean for the unique index scan. My understanding is that a unique
index scan would always return a single rowid.

Thank you,

D.

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