For FQSed query, run EXPLAIN on the remote node and print the result

We recently added fast-query-shipping facility to Postgres-XL. But it wouldn't
print the query plan used on the datanodes, which makes it extrememly difficult
to debug bad query plan with EXPLAIN. We now send an EXPLAIN query to one of
the datanodes and print the result. This assumes that all datanodes would plan
queries similarly, assuming a good data distribution.

We also modified the EXPLAIN output for FQSed queries to match is as closely as
possible with the Remote Subplan queries.

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index d17378692975fbe72d35bb9ea55c7809b1c64bb6..0f58ffb106d653fe3208416b932535cf36328726 100644 (file)
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -42,8 +42,10 @@
 #include "utils/xml.h"
 #ifdef PGXC
 #include "catalog/pgxc_node.h"
+#include "nodes/makefuncs.h"
 #include "pgxc/pgxcnode.h"
 #include "pgxc/planner.h"
+#include "pgxc/execRemote.h"
 #endif
 
 /* Hook for plugins to get control in ExplainOneQuery() */
@@ -995,7 +997,7 @@ ExplainNode(PlanState *planstate, List *ancestors,
                        break;
 #ifdef PGXC
                case T_RemoteQuery:
-                       pname = "Data Node Scan";
+                       pname = "Remote Fast Query Execution";
                        break;
 #endif
                case T_ForeignScan:
@@ -1156,7 +1158,6 @@ ExplainNode(PlanState *planstate, List *ancestors,
                case T_RemoteQuery:
                        /* Emit node execution list */
                        ExplainExecNodes(((RemoteQuery *)plan)->exec_nodes, es);
-                       ExplainScanTarget((Scan *) plan, es);
                        break;
 #endif
 #ifdef XCP
@@ -3411,6 +3412,103 @@ ExplainRemoteQuery(RemoteQuery *plan, PlanState *planstate, List *ancestors, Exp
        /* Remote query statement */
        if (es->verbose)
                ExplainPropertyText("Remote query", plan->sql_statement, es);
+
+       if (!es->analyze)
+       {
+               RemoteQuery *step = makeNode(RemoteQuery);
+               StringInfoData explainQuery;
+               StringInfoData explainResult;
+               EState *estate;
+               RemoteQueryState *node;
+               Var *dummy;
+               MemoryContext oldcontext;
+               TupleTableSlot *result;
+               bool firstline = true;
+
+               initStringInfo(&explainQuery);
+               initStringInfo(&explainResult);
+
+               if (es->format == EXPLAIN_FORMAT_TEXT)
+               {
+                       if (es->indent)
+                       {
+                               appendStringInfoSpaces(es->str, es->indent * 2);
+                               appendStringInfoString(es->str, "->  ");
+                               es->indent += 2;
+                       }
+               }
+
+               appendStringInfo(&explainQuery, "EXPLAIN (");
+               switch (es->format)
+               {
+                       case EXPLAIN_FORMAT_TEXT:
+                               appendStringInfo(&explainQuery, "FORMAT TEXT");
+                               break;
+                       case EXPLAIN_FORMAT_YAML:
+                               appendStringInfo(&explainQuery, "FORMAT YAML");
+                               break;
+                       case EXPLAIN_FORMAT_JSON:
+                               appendStringInfo(&explainQuery, "FORMAT JSON");
+                               break;
+                       case EXPLAIN_FORMAT_XML:
+                               appendStringInfo(&explainQuery, "FORMAT XML");
+                               break;
+               }
+               appendStringInfo(&explainQuery, ", VERBOSE %s", es->verbose ? "ON" : "OFF");
+               appendStringInfo(&explainQuery, ", COSTS %s", es->costs ? "ON" : "OFF");
+               appendStringInfo(&explainQuery, ", TIMING %s", es->timing ? "ON" : "OFF");
+               appendStringInfo(&explainQuery, ", BUFFERS %s", es->buffers ? "ON" : "OFF");
+               appendStringInfo(&explainQuery, ") %s", plan->sql_statement);
+
+               step->sql_statement = explainQuery.data;
+               step->combine_type = COMBINE_TYPE_NONE;
+               step->exec_nodes = copyObject(plan->exec_nodes);
+               step->exec_nodes->primarynodelist = NIL;
+
+               if (plan->exec_nodes && plan->exec_nodes->nodeList)
+                       step->exec_nodes->nodeList =
+                               list_make1_int(linitial_int(plan->exec_nodes->nodeList));
+
+               step->force_autocommit = true;
+               step->exec_type = EXEC_ON_DATANODES;
+
+               dummy = makeVar(1, 1, TEXTOID, -1, InvalidOid, 0);
+               plan->scan.plan.targetlist = lappend(plan->scan.plan.targetlist,
+                               makeTargetEntry((Expr *) dummy, 1, "QUERY PLAN", false));
+
+               /* Execute query on the data nodes */
+               estate = CreateExecutorState();
+
+               oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
+
+               estate->es_snapshot = GetActiveSnapshot();
+
+               node = ExecInitRemoteQuery(step, estate, 0);
+               MemoryContextSwitchTo(oldcontext);
+               result = ExecRemoteQuery(node);
+               while (result != NULL && !TupIsNull(result))
+               {
+                       Datum   value;
+                       bool    isnull;
+                       value = slot_getattr(result, 1, &isnull);
+                       if (!isnull)
+                       {
+                               if (!firstline)
+                                       appendStringInfoSpaces(&explainResult, 2 * es->indent);
+                               appendStringInfo(&explainResult, "%s\n", TextDatumGetCString(value));
+                               firstline = false;
+                       }
+
+                       /* fetch next */
+                       result = ExecRemoteQuery(node);
+               }
+               ExecEndRemoteQuery(node);
+
+               if (es->format == EXPLAIN_FORMAT_TEXT)
+                       appendStringInfo(es->str, "%s", explainResult.data);
+               else
+                       ExplainPropertyText("Remote plan", explainResult.data, es);
+       }
 }
 #endif