Previously, the view contents could become inconsistent with the base tables
in the following scenarios:
1) A concurrent transaction modifies a base table and commits before the
incremental view maintenance starts in the current transaction.
2) A concurrent transaction modifies a base table and commits before the
create_immv or refresh_immv command generates data.
3) Concurrent transactions incrementally update a view with a self-join
or modify multiple base tables simultaneously.
Incremental updates of a view are generally performed sequentially using an
exclusive lock. However, even if we are able to acquire the lock, a concurrent
transaction may have already incrementally updated the view and been committed
before we can acquire it. In REPEATABLE READ or SERIALIZABLE isolation levels,
this could lead to an inconsistent view state, which is the cause of the first
issue.
To fix this, a new field, lastivmupdate, has been added to the pg_ivm_immv
catalog to record the transaction ID of the most recent update to the view.
Before performing view maintenance, the transaction ID is checked. If the
transaction was still in progress at the start of the current transaction,
an error is raised to prevent anomalies.
To fix the second issue, the timing of CreateTrigger() has been moved to
before data generation. This ensures that locks conflicting with table
modifications have been acquired on all base tables. In addition, the latest
snapshot is used in READ COMMITTED level during the data generation to reflect
committed changes from concurrent transactions. Additionally, inconsistencies
that cannot be avoided through locking are prevented by checking the transaction
ID of the last view update, as done for the first issue.
However, concurrent table modifications and create_immv execution still cannot
be detected at the time of view creation. Therefore, create_immv raises a warning
in REPEATABLE READ or SERIALIZABLE isolation levels, suggesting that the command
be used in READ COMMITTED mode or that refresh_immv be executed afterward to
ensure the view remains consistent.
The third issue was caused by the snapshot used for checking tuple visibility in
the table's pre-update state not being the latest one. To fix this, the latest
snapshot is now used in READ COMMITTED mode.
Isolation tests are also added.
Issue #104
Previously, pg_upgrade failed due to the permission denied
because the pg_ivm_immv catalog was in the pg_catalog catalog
(Issue #79). To fix this, all objects created by pg_ivm are
moved to theschema pgivm, which is also created by pg_ivm.
pg_ivm is still not relocatable and this must be installed
to the pgivm schema because the catalog and some internal
functions are referred to unqualified by the schema name
from the pg_ivm module. In future, this might be able to
relocatable during installation, though.
This commit affects compatibility with previous releases.
To allow to access objects like create_immv function as
previous, you need to qualify them with the schema name
or setup search_path properly.
Simple CTEs which does not contain aggregates or DISTINCT are
now supported similarly to simple sub-queries.
Before a view is maintained, all CTEs are converted to corresponding
subqueries to enable to treat CTEs as same as subqueries. For this
end, codes of the static function inline_cte in the core
(optimizer/plan/subselect.c) was imported.
Prohibit Unreferenced CTE is prohibited.
When a table in a unreferenced CTE is TRUNCATEd, the contents
of the IMMV is not affected so it must not be truncated. For
confirming it at the maintenance time, we have to check if the
modified table used in a CTE is actually referenced. Although
it would possible, we just disallow to create such IMMVs for now
since such unreferenced CTE is useless unless it doesn't contain
modifying commands, that is already prohibited.
In order to re-calculate min/max values for groups where the min
or max value is deleted, we need the view query definition in string
form. However, pg_get_viewdef cannot be used for this purpose because
IMMV's defenition is in pg_ivm_immv but not pg_rewrite. Therefore,
we have to convert query definition in pg_ivm_immv to query
definition string. We can use pg_get_querydef in PG15, but we cannot
in PG14 or earlier, so we use codes in ruleutil.c copied from PG13
or PG14 depending versions.
refresh_immv(immv_name, with_data) is a function to refresh IMMV like
REFRESH MATERIALIZED VIEW command. It has two argument.
immv_name is incrementally maintainable materialized view's name, and
with_data is an option that is corresponding to the WITH [NO] DATA option.
When with_data is set false, the IMMV gets unpopulated.
One of differences between IMMVs unpopulated by this function and
normal materialized views unpopulated by REFRESH ... WITH NO DATA
is that such IMMVs can be referenced by SELECT but return no rows,
while unpopulated materialized views are not scanable.
The behaviour may be changed in future to raise an error when unpopulated
an IMMV is scanned.
