Some badly needed documentation about EvalPlanQual.

src/backend/executor/README

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+$Header: /cvsroot/pgsql/src/backend/executor/README,v 1.1 2001/05/15 00:35:50 tgl Exp $
+
+The Postgres Executor
+---------------------
+
+The executor processes a tree of "plan nodes".  The plan tree is essentially
+a demand-pull pipeline of tuple processing operations.  Each node, when
+called, will produce the next tuple in its output sequence, or NULL if no
+more tuples are available.  If the node is not a primitive relation-scanning
+node, it will have child node(s) that it calls in turn to obtain input
+tuples.
+
+Refinements on this basic model include:
+
+* Choice of scan direction (forwards or backwards).  Caution: this is not
+currently well-supported.  It works for primitive scan nodes, but not very
+well for joins, aggregates, etc.
+
+* Rescan command to reset a node and make it generate its output sequence
+over again.
+
+* Parameters that can alter a node's results.  After adjusting a parameter,
+the rescan command must be applied to that node and all nodes above it.
+There is a moderately intelligent scheme to avoid rescanning nodes
+unnecessarily (for example, Sort does not rescan its input if no parameters
+of the input have changed, since it can just reread its stored sorted data).
+
+The plan tree concept implements SELECT directly: it is only necessary to
+deliver the top-level result tuples to the client, or insert them into
+another table in the case of INSERT ... SELECT.  (INSERT ... VALUES is
+handled similarly, but the plan tree is just a Result node with no source
+tables.)  For UPDATE, the plan tree selects the tuples that need to be
+updated (WHERE condition) and delivers a new calculated tuple value for each
+such tuple, plus a "junk" (hidden) tuple CTID identifying the target tuple.
+The executor's top level then uses this information to update the correct
+tuple.  DELETE is similar to UPDATE except that only a CTID need be
+delivered by the plan tree.
+
+XXX a great deal more documentation needs to be written here...
+
+
+EvalPlanQual (READ COMMITTED update checking)
+---------------------------------------------
+
+For simple SELECTs, the executor need only pay attention to tuples that are
+valid according to the snapshot seen by the current transaction (ie, they
+were inserted by a previously committed transaction, and not deleted by any
+previously committed transaction).  However, for UPDATE and DELETE it is not
+cool to modify or delete a tuple that's been modified by an open or
+concurrently-committed transaction.  If we are running in SERIALIZABLE
+isolation level then we just raise an error when this condition is seen to
+occur.  In READ COMMITTED isolation level, we must work a lot harder.
+
+The basic idea in READ COMMITTED mode is to take the modified tuple
+committed by the concurrent transaction (after waiting for it to commit,
+if need be) and re-evaluate the query qualifications to see if it would
+still meet the quals.  If so, we regenerate the updated tuple (if we are
+doing an UPDATE) from the modified tuple, and finally update/delete the
+modified tuple.  SELECT FOR UPDATE behaves similarly, except that its action
+is just to mark the modified tuple for update by the current transaction.
+
+To implement this checking, we actually re-run the entire query from scratch
+for each modified tuple, but with the scan node that sourced the original
+tuple set to return only the modified tuple, not the original tuple or any
+of the rest of the relation.  If this query returns a tuple, then the
+modified tuple passes the quals (and the query output is the suitably
+modified update tuple, if we're doing UPDATE).  If no tuple is returned,
+then the modified tuple fails the quals, so we ignore it and continue the
+original query.  (This is reasonably efficient for simple queries, but may
+be horribly slow for joins.  A better design would be nice; one thought for
+future investigation is to treat the tuple substitution like a parameter,
+so that we can avoid rescanning unrelated nodes.)
+
+Note a fundamental bogosity of this approach: if the relation containing
+the original tuple is being used in a self-join, the other instance(s) of
+the relation will be treated as still containing the original tuple, whereas
+logical consistency would demand that the modified tuple appear in them too.
+But we'd have to actually substitute the modified tuple for the original,
+while still returning all the rest of the relation, to ensure consistent
+answers.  Implementing this correctly is a task for future work.
+
+In UPDATE/DELETE, only the target relation needs to be handled this way,
+so only one special recheck query needs to execute at a time.  In SELECT FOR
+UPDATE, there may be multiple relations flagged FOR UPDATE, so it's possible
+that while we are executing a recheck query for one modified tuple, we will
+hit another modified tuple in another relation.  In this case we "stack up"
+recheck queries: a sub-recheck query is spawned in which both the first and
+second modified tuples will be returned as the only components of their
+relations.  (In event of success, all these modified tuples will be marked
+for update.)  Again, this isn't necessarily quite the right thing ... but in
+simple cases it works.  Potentially, recheck queries could get nested to the
+depth of the number of FOR UPDATE relations in the query.
+
+It should be noted also that UPDATE/DELETE expect at most one tuple to
+result from the modified query, whereas in the FOR UPDATE case it's possible
+for multiple tuples to result (since we could be dealing with a join in
+which multiple tuples join to the modified tuple).  We want FOR UPDATE to
+mark all relevant tuples, so we pass all tuples output by all the stacked
+recheck queries back to the executor toplevel for marking.