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Question about an explain plan.

From: <dimitris_at_cs.umb.edu>
Date: 21 Oct 2005 17:38:05 -0700
Message-ID: <1129941485.565870.288920@o13g2000cwo.googlegroups.com>


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. Received on Fri Oct 21 2005 - 19:38:05 CDT

Original text of this message

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