I received a private email report that our introductory MVCC
documentation is unclear about when a snapshot is taken. I have
adjusted the wording in the attached patch to be less precise about
snapshot timing. Snapshot timing is controlled by the session isolation
level, which I don't think we want to cover in this introductory
paragraph.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
new file mode 100644
index cefd323..b75519f
*** a/doc/src/sgml/mvcc.sgml
--- b/doc/src/sgml/mvcc.sgml
***************
*** 41,52 ****
for developers to manage concurrent access to data. Internally,
data consistency is maintained by using a multiversion
model (Multiversion Concurrency Control, <acronym>MVCC</acronym>).
! This means that while querying a database each transaction sees
a snapshot of data (a <firstterm>database version</firstterm>)
as it was some
time ago, regardless of the current state of the underlying data.
! This protects the transaction from viewing inconsistent data that
! could be caused by (other) concurrent transaction updates on the same
data rows, providing <firstterm>transaction isolation</firstterm>
for each database session. <acronym>MVCC</acronym>, by eschewing
the locking methodologies of traditional database systems,
--- 41,52 ----
for developers to manage concurrent access to data. Internally,
data consistency is maintained by using a multiversion
model (Multiversion Concurrency Control, <acronym>MVCC</acronym>).
! This means that while querying a database each session sees
a snapshot of data (a <firstterm>database version</firstterm>)
as it was some
time ago, regardless of the current state of the underlying data.
! This prevents sessions from viewing inconsistent data produced
! by concurrent transactions performing updates on the same
data rows, providing <firstterm>transaction isolation</firstterm>
for each database session. <acronym>MVCC</acronym>, by eschewing
the locking methodologies of traditional database systems,
--
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