mirror of https://github.com/citusdata/citus.git
refactor local_executor.c lines with >78 characters
Onur Tirtir
parent
6dbb48c9f1
commit
17d9b934c3
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@ -3,21 +3,22 @@
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*
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* The scope of the local execution is locally executing the queries on the
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* shards. In other words, local execution does not deal with any local tables
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* that are not on shards on the node that the query is being executed. In that sense,
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* the local executor is only triggered if the node has both the metadata and the
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* shards (e.g., only Citus MX worker nodes).
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* that are not on shards on the node that the query is being executed. In that
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* sense, the local executor is only triggered if the node has both the metadata
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* and the shards (e.g., only Citus MX worker nodes).
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*
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* The goal of the local execution is to skip the unnecessary network round-trip
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* happening on the node itself. Instead, identify the locally executable tasks and
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* simply call PostgreSQL's planner and executor.
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* happening on the node itself. Instead, identify the locally executable tasks
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* and simply call PostgreSQL's planner and executor.
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*
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* The local executor is an extension of the adaptive executor. So, the executor uses
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* adaptive executor's custom scan nodes.
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* The local executor is an extension of the adaptive executor. So, the executor
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* uses adaptive executor's custom scan nodes.
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*
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* One thing to note is that Citus MX is only supported with replication factor = 1, so
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* keep that in mind while continuing the comments below.
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* One thing to note is that Citus MX is only supported with replication factor
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* to be equal to 1, so keep that in mind while continuing the comments below.
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*
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* On the high level, there are 3 slightly different ways of utilizing local execution:
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* On the high level, there are 3 slightly different ways of utilizing local
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* execution:
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*
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* (1) Execution of local single shard queries of a distributed table
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*
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@ -25,16 +26,19 @@
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* executor, and since the query is only a single task the execution finishes
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* without going to the network at all.
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*
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* Even if there is a transaction block (or recursively planned CTEs), as long
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* as the queries hit the shards on the same node, the local execution will kick in.
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* Even if there is a transaction block (or recursively planned CTEs), as
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* long as the queries hit the shards on the same node, the local execution
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* will kick in.
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*
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* (2) Execution of local single queries and remote multi-shard queries
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*
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* The rule is simple. If a transaction block starts with a local query execution,
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* all the other queries in the same transaction block that touch any local shard
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* have to use the local execution. Although this sounds restrictive, we prefer to
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* implement it in this way, otherwise we'd end-up with as complex scenarios as we
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* have in the connection managements due to foreign keys.
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* The rule is simple. If a transaction block starts with a local query
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* execution,
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* all the other queries in the same transaction block that touch any local
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* shard have to use the local execution. Although this sounds restrictive,
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* we prefer to implement it in this way, otherwise we'd end-up with as
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* complex scenarios as we have in the connection managements due to foreign
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* keys.
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*
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* See the following example:
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* BEGIN;
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@ -49,26 +53,28 @@
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*
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* (3) Modifications of reference tables
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*
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* Modifications to reference tables have to be executed on all nodes. So, after the
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* local execution, the adaptive executor keeps continuing the execution on the other
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* nodes.
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* Modifications to reference tables have to be executed on all nodes. So,
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* after the local execution, the adaptive executor keeps continuing the
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* execution on the other nodes.
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*
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* Note that for read-only queries, after the local execution, there is no need to
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* kick in adaptive executor.
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* Note that for read-only queries, after the local execution, there is no
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* need to kick in adaptive executor.
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*
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* There are also a few limitations/trade-offs that are worth mentioning.
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* - The local execution on multiple shards might be slow because the execution has to
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* happen one task at a time (e.g., no parallelism).
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* - If a transaction block/CTE starts with a multi-shard command, we do not use local query execution
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* since local execution is sequential. Basically, we do not want to lose parallelism
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* across local tasks by switching to local execution.
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* - The local execution currently only supports queries. In other words, any utility commands like TRUNCATE,
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* fails if the command is executed after a local execution inside a transaction block.
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* - The local execution on multiple shards might be slow because the execution
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* has to happen one task at a time (e.g., no parallelism).
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* - If a transaction block/CTE starts with a multi-shard command, we do not
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* use local query execution since local execution is sequential. Basically,
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* we do not want to lose parallelism across local tasks by switching to local
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* execution.
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* - The local execution currently only supports queries. In other words, any
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* utility commands like TRUNCATE, fails if the command is executed after a local
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* execution inside a transaction block.
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* - The local execution cannot be mixed with the executors other than adaptive,
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* namely task-tracker executor.
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* - Related with the previous item, COPY command
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* cannot be mixed with local execution in a transaction. The implication of that is any
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* part of INSERT..SELECT via coordinator cannot happen via the local execution.
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* - Related with the previous item, COPY command cannot be mixed with local
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* execution in a transaction. The implication of that is any part of INSERT..SELECT
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* via coordinator cannot happen via the local execution.
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*/
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#include "postgres.h"
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#include "miscadmin.h"
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@ -215,9 +221,10 @@ ExecuteLocalTaskList(CitusScanState *scanState, List *taskList)
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/*
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* ExtractParametersForLocalExecution extracts parameter types and values from
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* the given ParamListInfo structure, and fills parameter type and value arrays.
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* It does not change the oid of custom types, because the query will be run locally.
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* ExtractParametersForLocalExecution extracts parameter types and values
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* from the given ParamListInfo structure, and fills parameter type and
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* value arrays. It does not change the oid of custom types, because the
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* query will be run locally.
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*/
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static void
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ExtractParametersForLocalExecution(ParamListInfo paramListInfo, Oid **parameterTypes,
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@ -272,8 +279,8 @@ ExtractLocalAndRemoteTasks(bool readOnly, List *taskList, List **localTaskList,
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List *localTaskPlacementList = NULL;
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List *remoteTaskPlacementList = NULL;
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SplitLocalAndRemotePlacements(task->taskPlacementList, &localTaskPlacementList,
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&remoteTaskPlacementList);
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SplitLocalAndRemotePlacements(
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task->taskPlacementList, &localTaskPlacementList, &remoteTaskPlacementList);
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/* either the local or the remote should be non-nil */
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Assert(!(localTaskPlacementList == NIL && remoteTaskPlacementList == NIL));
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@ -281,9 +288,9 @@ ExtractLocalAndRemoteTasks(bool readOnly, List *taskList, List **localTaskList,
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if (list_length(task->taskPlacementList) == 1)
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{
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/*
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* At this point, the task has a single placement (e.g,. anchor shard is
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* distributed table's shard). So, it is either added to local or remote
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* taskList.
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* At this point, the task has a single placement (e.g,. anchor shard
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* is distributed table's shard). So, it is either added to local or
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* remote taskList.
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*/
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if (localTaskPlacementList == NIL)
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{
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@ -297,10 +304,10 @@ ExtractLocalAndRemoteTasks(bool readOnly, List *taskList, List **localTaskList,
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else
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{
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/*
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* At this point, we're dealing with reference tables or intermediate results
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* where the task has placements on both local and remote nodes. We always
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* prefer to use local placement, and require remote placements only for
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* modifications.
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* At this point, we're dealing with reference tables or intermediate
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* results where the task has placements on both local and remote
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* nodes. We always prefer to use local placement, and require remote
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* placements only for modifications.
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*/
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task->partiallyLocalOrRemote = true;
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@ -326,9 +333,9 @@ ExtractLocalAndRemoteTasks(bool readOnly, List *taskList, List **localTaskList,
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/*
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* SplitLocalAndRemotePlacements is a helper function which iterates over the input
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* taskPlacementList and puts the placements into corresponding list of either
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* localTaskPlacementList or remoteTaskPlacementList.
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* SplitLocalAndRemotePlacements is a helper function which iterates over the
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* input taskPlacementList and puts the placements into corresponding list of
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* either localTaskPlacementList or remoteTaskPlacementList.
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*/
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static void
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SplitLocalAndRemotePlacements(List *taskPlacementList, List **localTaskPlacementList,
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@ -433,8 +440,8 @@ ShouldExecuteTasksLocally(List *taskList)
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/*
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* TODO: A future improvement could be to keep track of which placements
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* have been locally executed. At this point, only use local execution for
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* those placements. That'd help to benefit more from parallelism.
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* have been locally executed. At this point, only use local execution
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* for those placements. That'd help to benefit more from parallelism.
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*/
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return true;
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@ -446,14 +453,14 @@ ShouldExecuteTasksLocally(List *taskList)
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{
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/*
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* This is the valuable time to use the local execution. We are likely
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* to avoid any network round-trips by simply executing the command within
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* this session.
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* to avoid any network round-trips by simply executing the command
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* within this session.
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*
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* We cannot avoid network round trips if the task is not a read only
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* task and accesses multiple placements. For example, modifications to
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* distributed tables (with replication factor == 1) would avoid network
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* round-trips. However, modifications to reference tables still needs to
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* go to over the network to do the modification on the other placements.
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* round-trips. However, modifications to reference tables still needs
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* to go to over the network to do the modification on the other placements.
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* Still, we'll be avoding the network round trip for this node.
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*
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* Note that we shouldn't use local execution if any distributed execution
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@ -466,10 +473,10 @@ ShouldExecuteTasksLocally(List *taskList)
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if (!singleTask)
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{
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/*
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* For multi-task executions, switching to local execution would likely to
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* perform poorly, because we'd lose the parallelism. Note that the local
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* execution is happening one task at a time (e.g., similar to sequential
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* distributed execution).
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* For multi-task executions, switching to local execution would likely
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* to perform poorly, because we'd lose the parallelism. Note that the
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* local execution is happening one task at a time (e.g., similar to
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* sequential distributed execution).
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*/
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Assert(!TransactionAccessedLocalPlacement);
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@ -481,8 +488,8 @@ ShouldExecuteTasksLocally(List *taskList)
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/*
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* TaskAccessesLocalNode returns true if any placements of the task reside on the
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* node that we're executing the query.
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* TaskAccessesLocalNode returns true if any placements of the task reside on
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* the node that we're executing the query.
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*/
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bool
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TaskAccessesLocalNode(Task *task)
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@ -503,24 +510,26 @@ TaskAccessesLocalNode(Task *task)
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/*
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* ErrorIfTransactionAccessedPlacementsLocally() errors out if a local query on any shard
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* has already been executed in the same transaction.
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* ErrorIfTransactionAccessedPlacementsLocally errors out if a local query
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* on any shard has already been executed in the same transaction.
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*
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* This check is required because Citus currently hasn't implemented local execution
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* infrastructure for all the commands/executors. As we implement local execution for
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* the command/executor that this function call exists, we should simply remove the check.
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* This check is required because Citus currently hasn't implemented local
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* execution infrastructure for all the commands/executors. As we implement
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* local execution for the command/executor that this function call exists,
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* we should simply remove the check.
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*/
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void
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ErrorIfTransactionAccessedPlacementsLocally(void)
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{
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if (TransactionAccessedLocalPlacement)
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{
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ereport(ERROR, (errmsg("cannot execute command because a local execution has "
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"accessed a placement in the transaction"),
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errhint("Try re-running the transaction with "
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"\"SET LOCAL citus.enable_local_execution TO OFF;\""),
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errdetail("Some parallel commands cannot be executed if a "
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"previous command has already been executed locally")));
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ereport(ERROR,
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(errmsg("cannot execute command because a local execution has "
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"accessed a placement in the transaction"),
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errhint("Try re-running the transaction with "
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"\"SET LOCAL citus.enable_local_execution TO OFF;\""),
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errdetail("Some parallel commands cannot be executed if a "
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"previous command has already been executed locally")));
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}
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}
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