Add infrastructure to support EphemeralNamedRelation references.

A QueryEnvironment concept is added, which allows new types of
objects to be passed into queries from parsing on through
execution.  At this point, the only thing implemented is a
collection of EphemeralNamedRelation objects -- relations which
can be referenced by name in queries, but do not exist in the
catalogs.  The only type of ENR implemented is NamedTuplestore, but
provision is made to add more types fairly easily.

An ENR can carry its own TupleDesc or reference a relation in the
catalogs by relid.

Although these features can be used without SPI, convenience
functions are added to SPI so that ENRs can easily be used by code
run through SPI.

The initial use of all this is going to be transition tables in
AFTER triggers, but that will be added to each PL as a separate
commit.

An incidental effect of this patch is to produce a more informative
error message if an attempt is made to modify the contents of a CTE
from a referencing DML statement.  No tests previously covered that
possibility, so one is added.

Kevin Grittner and Thomas Munro
Reviewed by Heikki Linnakangas, David Fetter, and Thomas Munro
with valuable comments and suggestions from many others

1 files changed, 144 insertions, 0 deletions

diff --git a/src/backend/utils/misc/queryenvironment.c b/src/backend/utils/misc/queryenvironment.c
new file mode 100644
index 00000000000..a0b10d402bd
--- /dev/null
+++ b/src/backend/utils/misc/queryenvironment.c
@@ -0,0 +1,144 @@
+/*-------------------------------------------------------------------------
+ *
+ * queryenvironment.c
+ *	  Query environment, to store context-specific values like ephemeral named
+ *	  relations.  Initial use is for named tuplestores for delta information
+ *	  from "normal" relations.
+ *
+ * The initial implementation uses a list because the number of such relations
+ * in any one context is expected to be very small.  If that becomes a
+ * performance problem, the implementation can be changed with no other impact
+ * on callers, since this is an opaque structure.  This is the reason to
+ * require a create function.
+ *
+ * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/backend/utils/misc/queryenvironment.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/heapam.h"
+#include "utils/queryenvironment.h"
+#include "utils/rel.h"
+
+/*
+ * Private state of a query environment.
+ */
+struct QueryEnvironment
+{
+	List	   *namedRelList;
+};
+
+
+QueryEnvironment *
+create_queryEnv()
+{
+	return (QueryEnvironment *) palloc0(sizeof(QueryEnvironment));
+}
+
+EphemeralNamedRelationMetadata
+get_visible_ENR_metadata(QueryEnvironment *queryEnv, const char *refname)
+{
+	EphemeralNamedRelation enr;
+
+	Assert(refname != NULL);
+
+	if (queryEnv == NULL)
+		return NULL;
+
+	enr = get_ENR(queryEnv, refname);
+
+	if (enr)
+		return &(enr->md);
+
+	return NULL;
+}
+
+/*
+ * Register a named relation for use in the given environment.
+ *
+ * If this is intended exclusively for planning purposes, the tstate field can
+ * be left NULL;
+ */
+void
+register_ENR(QueryEnvironment *queryEnv, EphemeralNamedRelation enr)
+{
+	Assert(enr != NULL);
+	Assert(get_ENR(queryEnv, enr->md.name) == NULL);
+
+	queryEnv->namedRelList = lappend(queryEnv->namedRelList, enr);
+}
+
+/*
+ * Unregister an ephemeral relation by name.  This will probably be a rarely
+ * used function, but seems like it should be provided "just in case".
+ */
+void
+unregister_ENR(QueryEnvironment *queryEnv, const char *name)
+{
+	EphemeralNamedRelation match;
+
+	match = get_ENR(queryEnv, name);
+	if (match)
+		queryEnv->namedRelList = list_delete(queryEnv->namedRelList, match);
+}
+
+/*
+ * This returns an ENR if there is a name match in the given collection.  It
+ * must quietly return NULL if no match is found.
+ */
+EphemeralNamedRelation
+get_ENR(QueryEnvironment *queryEnv, const char *name)
+{
+	ListCell   *lc;
+
+	Assert(name != NULL);
+
+	if (queryEnv == NULL)
+		return NULL;
+
+	foreach(lc, queryEnv->namedRelList)
+	{
+		EphemeralNamedRelation enr = (EphemeralNamedRelation) lfirst(lc);
+
+		if (strcmp(enr->md.name, name) == 0)
+			return enr;
+	}
+
+	return NULL;
+}
+
+/*
+ * Gets the TupleDesc for a Ephemeral Named Relation, based on which field was
+ * filled.
+ *
+ * When the TupleDesc is based on a relation from the catalogs, we count on
+ * that relation being used at the same time, so that appropriate locks will
+ * already be held.  Locking here would be too late anyway.
+ */
+TupleDesc
+ENRMetadataGetTupDesc(EphemeralNamedRelationMetadata enrmd)
+{
+	TupleDesc	tupdesc;
+
+	/* One, and only one, of these fields must be filled. */
+	Assert((enrmd->reliddesc == InvalidOid) != (enrmd->tupdesc == NULL));
+
+	if (enrmd->tupdesc != NULL)
+		tupdesc = enrmd->tupdesc;
+	else
+	{
+		Relation	relation;
+
+		relation = heap_open(enrmd->reliddesc, NoLock);
+		tupdesc = relation->rd_att;
+		heap_close(relation, NoLock);
+	}
+
+	return tupdesc;
+}