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Add a Gather Merge executor node.
Like Gather, we spawn multiple workers and run the same plan in each one; however, Gather Merge is used when each worker produces the same output ordering and we want to preserve that output ordering while merging together the streams of tuples from various workers. (In a way, Gather Merge is like a hybrid of Gather and MergeAppend.) This works out to a win if it saves us from having to perform an expensive Sort. In cases where only a small amount of data would need to be sorted, it may actually be faster to use a regular Gather node and then sort the results afterward, because Gather Merge sometimes needs to wait synchronously for tuples whereas a pure Gather generally doesn't. But if this avoids an expensive sort then it's a win. Rushabh Lathia, reviewed and tested by Amit Kapila, Thomas Munro, and Neha Sharma, and reviewed and revised by me. Discussion: http://postgr.es/m/CAGPqQf09oPX-cQRpBKS0Gq49Z+m6KBxgxd_p9gX8CKk_d75HoQ@mail.gmail.com
1 parent a72f036 commit 355d399

27 files changed

+1355
-16
lines changed

‎doc/src/sgml/config.sgml

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@@ -3497,6 +3497,20 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
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</listitem>
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</varlistentry>
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<varlistentry id="guc-enable-gathermerge" xreflabel="enable_gathermerge">
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<term><varname>enable_gathermerge</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>enable_gathermerge</> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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Enables or disables the query planner's use of gather
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merge plan types. The default is <literal>on</>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
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<term><varname>enable_hashagg</varname> (<type>boolean</type>)
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<indexterm>

‎src/backend/commands/explain.c

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@@ -918,6 +918,9 @@ ExplainNode(PlanState *planstate, List *ancestors,
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case T_Gather:
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pname = sname = "Gather";
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break;
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case T_GatherMerge:
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pname = sname = "Gather Merge";
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break;
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case T_IndexScan:
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pname = sname = "Index Scan";
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break;
@@ -1411,6 +1414,26 @@ ExplainNode(PlanState *planstate, List *ancestors,
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ExplainPropertyBool("Single Copy", gather->single_copy, es);
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}
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break;
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case T_GatherMerge:
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{
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GatherMerge *gm = (GatherMerge *) plan;
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show_scan_qual(plan->qual, "Filter", planstate, ancestors, es);
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if (plan->qual)
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show_instrumentation_count("Rows Removed by Filter", 1,
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planstate, es);
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ExplainPropertyInteger("Workers Planned",
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gm->num_workers, es);
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if (es->analyze)
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{
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int nworkers;
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nworkers = ((GatherMergeState *) planstate)->nworkers_launched;
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ExplainPropertyInteger("Workers Launched",
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nworkers, es);
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}
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}
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break;
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case T_FunctionScan:
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if (es->verbose)
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{

‎src/backend/executor/Makefile

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@@ -20,7 +20,7 @@ OBJS = execAmi.o execCurrent.o execGrouping.o execIndexing.o execJunk.o \
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nodeBitmapHeapscan.o nodeBitmapIndexscan.o \
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nodeCustom.o nodeFunctionscan.o nodeGather.o \
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nodeHash.o nodeHashjoin.o nodeIndexscan.o nodeIndexonlyscan.o \
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nodeLimit.o nodeLockRows.o \
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nodeLimit.o nodeLockRows.o nodeGatherMerge.o \
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nodeMaterial.o nodeMergeAppend.o nodeMergejoin.o nodeModifyTable.o \
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nodeNestloop.o nodeProjectSet.o nodeRecursiveunion.o nodeResult.o \
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nodeSamplescan.o nodeSeqscan.o nodeSetOp.o nodeSort.o nodeUnique.o \

‎src/backend/executor/execProcnode.c

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@@ -89,6 +89,7 @@
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#include "executor/nodeForeignscan.h"
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#include "executor/nodeFunctionscan.h"
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#include "executor/nodeGather.h"
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#include "executor/nodeGatherMerge.h"
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#include "executor/nodeGroup.h"
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#include "executor/nodeHash.h"
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#include "executor/nodeHashjoin.h"
@@ -326,6 +327,11 @@ ExecInitNode(Plan *node, EState *estate, int eflags)
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estate, eflags);
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break;
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case T_GatherMerge:
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result = (PlanState *) ExecInitGatherMerge((GatherMerge *) node,
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estate, eflags);
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break;
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case T_Hash:
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result = (PlanState *) ExecInitHash((Hash *) node,
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estate, eflags);
@@ -535,6 +541,10 @@ ExecProcNode(PlanState *node)
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result = ExecGather((GatherState *) node);
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break;
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case T_GatherMergeState:
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result = ExecGatherMerge((GatherMergeState *) node);
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break;
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case T_HashState:
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result = ExecHash((HashState *) node);
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break;
@@ -697,6 +707,10 @@ ExecEndNode(PlanState *node)
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ExecEndGather((GatherState *) node);
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break;
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case T_GatherMergeState:
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ExecEndGatherMerge((GatherMergeState *) node);
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break;
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case T_IndexScanState:
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ExecEndIndexScan((IndexScanState *) node);
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break;
@@ -842,6 +856,9 @@ ExecShutdownNode(PlanState *node)
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case T_CustomScanState:
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ExecShutdownCustomScan((CustomScanState *) node);
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break;
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case T_GatherMergeState:
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ExecShutdownGatherMerge((GatherMergeState *) node);
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break;
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default:
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break;
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}

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