number of rows in EXPLAIN for unpacked vs packed tables 2006-06-30 - By Dan Buettner
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Cor -
Those numbers are an estimate, not a hard number, of how many rows
MySQL thinks it will have to exmaine to find what you're looking for.
They could be different because your compressed table is brand-new and
all the indexes are clean and up to date, whereas your original table
has possibly been inserted, updated, and deleted from, causing the key
information to be less accurate.
You could try running a CHECK TABLE <tablename> EXTENDED on the old
one, which should update all the key info, and then checking your
EXPLAIN results again. Be aware it could take a while for a large
table and will lock the table for the duration.
If your indexes are different on the compressed table that would make
a difference too, as MySQL might well be basing its estimate off a
different index.
Dan
On 6/30/06, C.R.Vegelin <cr.vegelin@(protected)> wrote:
> Hi All,
>
> I compressed a MyISAM table successfully with MYISAMPACK, followed by
MYISAMCHK.
> Both tables (MyISAM + Compressed ) have exactly the same number of rows with
Count(*).
> But when I give a SELECT query with EXPLAIN on both tables, I get different
number of rows.
> For example: EXPLAIN SELECT ... FROM data ... shows 1983 rows,
> but the same EXPLAIN SELECT ... FROM pack ... shows 2064 rows.
> Any idea why ?
>
> Regards, Cor
>
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