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Update executor documentation for run-time partition pruning 

With run-time partition pruning, there is no longer necessarily an
executor node for each corresponding plan node.

Author: David Rowley <david.rowley@2ndquadrant.com>
This commit is contained in:
Peter Eisentraut 2018-11-15 23:02:21 +01:00
parent 8fbb2a92e0
commit fd5274a65b
1 changed files with 15 additions and 7 deletions
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22
src/backend/executor/README
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@ -45,13 +45,21 @@ Plan Trees and State Trees
The plan tree delivered by the planner contains a tree of Plan nodes (struct
types derived from struct Plan). During executor startup we build a parallel
tree of identical structure containing executor state nodes --- every plan
node type has a corresponding executor state node type. Each node in the
state tree has a pointer to its corresponding node in the plan tree, plus
executor state data as needed to implement that node type. This arrangement
allows the plan tree to be completely read-only so far as the executor is
concerned: all data that is modified during execution is in the state tree.
Read-only plan trees make life much simpler for plan caching and reuse.
tree of identical structure containing executor state nodes --- generally,
every plan node type has a corresponding executor state node type. Each node
in the state tree has a pointer to its corresponding node in the plan tree,
plus executor state data as needed to implement that node type. This
arrangement allows the plan tree to be completely read-only so far as the
executor is concerned: all data that is modified during execution is in the
state tree. Read-only plan trees make life much simpler for plan caching and
reuse.
A corresponding executor state node may not be created during executor startup
if the executor determines that an entire subplan is not required due to
execution time partition pruning determining that no matching records will be
found there. This currently only occurs for Append and MergeAppend nodes. In
this case the non-required subplans are ignored and the executor state's
subnode array will become out of sequence to the plan's subplan list.
Each Plan node may have expression trees associated with it, to represent
its target list, qualification conditions, etc. These trees are also