Add support for ON CONFLICT DO SELECT [ FOR ... ]

Adds support for DO SELECT action for ON CONFLICT clause where we
select the tuples and optionally lock them. If the tuples are locked
with check for conflicts, otherwise not.

Author: jeltz

doc/src/sgml/ref/insert.sgml

@@ -37,6 +37,7 @@ INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replac
 <phrase>and <replaceable class="parameter">conflict_action</replaceable> is one of:</phrase>
 
     DO NOTHING
+    DO SELECT [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } ]
     DO UPDATE SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } |
                     ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = [ ROW ] ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) |
                     ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( <replaceable class="parameter">sub-SELECT</replaceable> )
@@ -88,18 +89,24 @@ INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replac
 
   <para>
    The optional <literal>RETURNING</literal> clause causes <command>INSERT</command>
-   to compute and return value(s) based on each row actually inserted
-   (or updated, if an <literal>ON CONFLICT DO UPDATE</literal> clause was
-   used).  This is primarily useful for obtaining values that were
+   to compute and return value(s) based on each row actually inserted.
+   If an <literal>ON CONFLICT DO UPDATE</literal> clause was used,
+   <literal>RETURNING</literal> also returns tuples which were updated, and
+   in the presence of an <literal>ON CONFLICT DO SELECT</literal> clause all
+   input rows are returned.  With a traditional <command>INSERT</command>,
+   the <literal>RETURNING</literal> clause is primarily useful for obtaining
+   values that were
    supplied by defaults, such as a serial sequence number.  However,
    any expression using the table's columns is allowed.  The syntax of
    the <literal>RETURNING</literal> list is identical to that of the output
-   list of <command>SELECT</command>.  Only rows that were successfully
+   list of <command>SELECT</command>.  If an <literal>ON CONFLICT DO SELECT</literal>
+   clause is not present, only rows that were successfully
    inserted or updated will be returned.  For example, if a row was
    locked but not updated because an <literal>ON CONFLICT DO UPDATE
    ... WHERE</literal> clause <replaceable
    class="parameter">condition</replaceable> was not satisfied, the
-   row will not be returned.
+   row will not be returned.  <literal>ON CONFLICT DO SELECT</literal>
+   works similarly, except no update takes place.
   </para>
 
   <para>

src/backend/commands/explain.c

@@ -4651,10 +4651,35 @@ show_modifytable_info(ModifyTableState *mtstate, List *ancestors,
 
 	if (node->onConflictAction != ONCONFLICT_NONE)
 	{
-		ExplainPropertyText("Conflict Resolution",
-							node->onConflictAction == ONCONFLICT_NOTHING ?
-							"NOTHING" : "UPDATE",
-							es);
+		const char *resolution;
+
+		if (node->onConflictAction == ONCONFLICT_NOTHING)
+			resolution = "NOTHING";
+		else if (node->onConflictAction == ONCONFLICT_UPDATE)
+			resolution = "UPDATE";
+		else
+		{
+			switch (node->onConflictLockingStrength)
+			{
+				case LCS_NONE:
+					resolution = "SELECT";
+					break;
+				case LCS_FORKEYSHARE:
+					resolution = "SELECT FOR KEY SHARE";
+					break;
+				case LCS_FORSHARE:
+					resolution = "SELECT FOR SHARE";
+					break;
+				case LCS_FORNOKEYUPDATE:
+					resolution = "SELECT FOR NO KEY UPDATE";
+					break;
+				default:		/* LCS_FORUPDATE */
+					resolution = "SELECT FOR UPDATE";
+					break;
+			}
+		}
+
+		ExplainPropertyText("Conflict Resolution", resolution, es);
 
 		/*
 		 * Don't display arbiter indexes at all when DO NOTHING variant