Hello Martijn, hello List,
Thank you very much.
I still have some questions, further to your answers :
Martijn van Oosterhout wrote:
On Fri, Jul 28, 2006 at 12:00:19PM +0200, DANTE Alexandra wrote:
Hello List,
I try to understand perfectly the mecanisms used to update / delete a
tuple (and consequently those used in VACUUM) and I've got some questions.
<snip example>
I hope someone could answer these questions :
- what are the new values for xmin, xmax and ctid for an updated tuple ?
xmin is the transaction that created the tuple (ie your XID)
xmax is the transaction that deleted the tuple (ie zero, it's not dead)
ctid is wherever it ends up on disk
- what about the old tuple ? what is the value for xmax ?
Your XID, given you deleted it.
- is it correct to think that the ctid of the old version of the tuple
is a link to newer version ? In my example, is it correct to think that
the tuple :
140049 | 0 | (0,12) | 11 | IRAQ
has become :
new value | 0 | (0,26) | 11 | ITALY
The word "become" is not really right. The old version has become
invisible to you and the new version is visible. Some other
transactions will see the old one, some the new one. However, there is
a link between the old and the new version do detect conflicting
updates.
I've just seen that I've done a mistake in my example. My question was :
is it correct to think that the ctid of the old version of the tuple is
a link to newer version ? In my example, is it correct to think that the
tuple :
140049 | 0 | (0,12) | 11 | IRAQ
has become :
new value | 0 | (0,26) | 11 | *IRAQ*
Could you give me more details about the link between the old and the
new version, please ?
For me, the link is the c_tid, but maybe I'm wrong...
- what are the values set in the "infomask" structure for the old
version of the tuple ?
I don't think there are any changes. Until your transaction commits you
can't really know if the tuple is really deleted or not. The first
transaction to read the tuple after your transaction commits will
update the bits.
My question about the "infomask" strucutre was linked to the code of
VACUUM. I've seen in the "lazy_scan_heap method that the
"HeapTupleSatisfiesVacuum" method is called. In this method, according
to the value of "infomask", a tuple is defined as "dead" or not.
That's why I wonder if the "infomask" structure is changed after an
commited update or delete, and what are the values set ?
And then, after all these questions about tables, I've got questions
about index. Imagine that we have an index of the "n_name" column, after
the update :
- is it correct to think that a new index tuple has been created ?
Yes
- does the old index tuple link to the new index tuple ?
No
- if not, how the B-tree can be still balanced ? is it necessary to
rebuild the index ?
The b-tree code attempts to keep itself balanced. But it does nothing
special for an UPDATE, it works the same as an INSERT.
So, consequently, it is not necessary to rebuild the B-tree index after
an update or a delete.
Is it correct ?
Hope this helps,
Regards,
Alexandra
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