pg_ivm/pg_ivm--1.2--1.3.sql

-- functions

CREATE FUNCTION ivm_visible_in_prestate(oid, tid, oid)
RETURNS bool
STABLE
AS 'MODULE_PATHNAME', 'ivm_visible_in_prestate'
LANGUAGE C;
Use snapshot to check tuple visibility in pre-update state (#28) When multiple tables are updated or the view contains a self-join, we need to calculate table states that was before it is modified during incremental view maintenance. For get the pre-update state, tuples inserted in a command must be removed when the table is scanned. Previously, we used xmin and cmin system columns for this purpose, but this way is problematic because after a tuple is frozen, its xmin no longer has any meaning. Actually, we will get inconsistent view results after XID wraparound. Also, we can see similar similar inconsistency when using sub-transaction because xmin values do not always monotonically increasing by command executions. To fix this, we use a snapshot that taken just before the table is modified for checking tuple visibility in pre-state table instead of using xmin and cmin system columns. A new function returning boolean, ivm_visible_in_prestate, is added, and this is called in WHERE clause of sub-queries to calculate pre-state table. This function check if a specified tuple in the post-update table is visible or not using the snapshot and return true if the tuple is visible. 2022-09-30 02:53:12 +00:00			`-- functions`

			`CREATE FUNCTION ivm_visible_in_prestate(oid, tid, oid)`
			`RETURNS bool`
			`STABLE`
			`AS 'MODULE_PATHNAME', 'ivm_visible_in_prestate'`
			`LANGUAGE C;`