[PATCH] Document heuristics for parameterized paths

Hi,

A small patch to the documentation about how to reduce the number of
parameterized paths, because it kept me searching for a long time :-)
(The code this documents is in add_paths_to_joinrel(), the loop
foreach(lc, root->join_info_list).)

diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README
index 41c120e0cd..79e270188d 100644
--- a/src/backend/optimizer/README
+++ b/src/backend/optimizer/README
@@ -863,6 +863,19 @@ that. An exception occurs for parameterized paths for the RHS relation of
a SEMI or ANTI join: in those cases, we can stop the inner scan after the
first match, so it's primarily startup not total cost that we care about.

+Furthermore, join trees involving parameterized paths are kept as left-deep
+as possible; for nested loops consisting of inner joins only, bushy plans
+are equivalent to left-deep ones, so keeping bushy plans is only a waste.
+This is enforced by refusing to join parameterized paths together unless
+the parameterization is resolved, *or* the remaining parameterization is
+one that must cannot be delayed right away (because of outer join
+restrictions). This ensures that we do not keep around large subplans that
+are parameterized on a whole host of external relations, without losing
+any plans. An exception is that we are allowed to keep a parameterization
+around if we *partially* resolve it, i.e., we had a multi-part index and
+resolved only one table from it. This is known as the "star-schema"
+exception.
+

LATERAL subqueries
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