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diff --git a/src/backend/executor/README b/src/backend/executor/README new file mode 100644 index 0000000..bf5e708 --- /dev/null +++ b/src/backend/executor/README @@ -0,0 +1,405 @@ +src/backend/executor/README + +The Postgres Executor +===================== + +The executor processes a tree of "plan nodes". The plan tree is essentially +a demand-pull pipeline of tuple processing operations. Each node, when +called, will produce the next tuple in its output sequence, or NULL if no +more tuples are available. If the node is not a primitive relation-scanning +node, it will have child node(s) that it calls in turn to obtain input +tuples. + +Refinements on this basic model include: + +* Choice of scan direction (forwards or backwards). Caution: this is not +currently well-supported. It works for primitive scan nodes, but not very +well for joins, aggregates, etc. + +* Rescan command to reset a node and make it generate its output sequence +over again. + +* Parameters that can alter a node's results. After adjusting a parameter, +the rescan command must be applied to that node and all nodes above it. +There is a moderately intelligent scheme to avoid rescanning nodes +unnecessarily (for example, Sort does not rescan its input if no parameters +of the input have changed, since it can just reread its stored sorted data). + +For a SELECT, it is only necessary to deliver the top-level result tuples +to the client. For INSERT/UPDATE/DELETE, the actual table modification +operations happen in a top-level ModifyTable plan node. If the query +includes a RETURNING clause, the ModifyTable node delivers the computed +RETURNING rows as output, otherwise it returns nothing. Handling INSERT +is pretty straightforward: the tuples returned from the plan tree below +ModifyTable are inserted into the correct result relation. For UPDATE, +the plan tree returns the new values of the updated columns, plus "junk" +(hidden) column(s) identifying which table row is to be updated. The +ModifyTable node must fetch that row to extract values for the unchanged +columns, combine the values into a new row, and apply the update. (For a +heap table, the row-identity junk column is a CTID, but other things may +be used for other table types.) For DELETE, the plan tree need only deliver +junk row-identity column(s), and the ModifyTable node visits each of those +rows and marks the row deleted. + +XXX a great deal more documentation needs to be written here... + + +Plan Trees and State Trees +-------------------------- + +The plan tree delivered by the planner contains a tree of Plan nodes (struct +types derived from struct Plan). During executor startup we build a parallel +tree of identical structure containing executor state nodes --- generally, +every plan node type has a corresponding executor state node type. Each node +in the state tree has a pointer to its corresponding node in the plan tree, +plus executor state data as needed to implement that node type. This +arrangement allows the plan tree to be completely read-only so far as the +executor is concerned: all data that is modified during execution is in the +state tree. Read-only plan trees make life much simpler for plan caching and +reuse. + +A corresponding executor state node may not be created during executor startup +if the executor determines that an entire subplan is not required due to +execution time partition pruning determining that no matching records will be +found there. This currently only occurs for Append and MergeAppend nodes. In +this case the non-required subplans are ignored and the executor state's +subnode array will become out of sequence to the plan's subplan list. + +Each Plan node may have expression trees associated with it, to represent +its target list, qualification conditions, etc. These trees are also +read-only to the executor, but the executor state for expression evaluation +does not mirror the Plan expression's tree shape, as explained below. +Rather, there's just one ExprState node per expression tree, although this +may have sub-nodes for some complex expression node types. + +Altogether there are four classes of nodes used in these trees: Plan nodes, +their corresponding PlanState nodes, Expr nodes, and ExprState nodes. +(Actually, there are also List nodes, which are used as "glue" in all +three tree-based representations.) + + +Expression Trees and ExprState nodes +------------------------------------ + +Expression trees, in contrast to Plan trees, are not mirrored into a +corresponding tree of state nodes. Instead each separately executable +expression tree (e.g. a Plan's qual or targetlist) is represented by one +ExprState node. The ExprState node contains the information needed to +evaluate the expression in a compact, linear form. That compact form is +stored as a flat array in ExprState->steps[] (an array of ExprEvalStep, +not ExprEvalStep *). + +The reasons for choosing such a representation include: +- commonly the amount of work needed to evaluate one Expr-type node is + small enough that the overhead of having to perform a tree-walk + during evaluation is significant. +- the flat representation can be evaluated non-recursively within a single + function, reducing stack depth and function call overhead. +- such a representation is usable both for fast interpreted execution, + and for compiling into native code. + +The Plan-tree representation of an expression is compiled into an +ExprState node by ExecInitExpr(). As much complexity as possible should +be handled by ExecInitExpr() (and helpers), instead of execution time +where both interpreted and compiled versions would need to deal with the +complexity. Besides duplicating effort between execution approaches, +runtime initialization checks also have a small but noticeable cost every +time the expression is evaluated. Therefore, we allow ExecInitExpr() to +precompute information that we do not expect to vary across execution of a +single query, for example the set of CHECK constraint expressions to be +applied to a domain type. This could not be done at plan time without +greatly increasing the number of events that require plan invalidation. +(Previously, some information of this kind was rechecked on each +expression evaluation, but that seems like unnecessary overhead.) + + +Expression Initialization +------------------------- + +During ExecInitExpr() and similar routines, Expr trees are converted +into the flat representation. Each Expr node might be represented by +zero, one, or more ExprEvalSteps. + +Each ExprEvalStep's work is determined by its opcode (of enum ExprEvalOp) +and it stores the result of its work into the Datum variable and boolean +null flag variable pointed to by ExprEvalStep->resvalue/resnull. +Complex expressions are performed by chaining together several steps. +For example, "a + b" (one OpExpr, with two Var expressions) would be +represented as two steps to fetch the Var values, and one step for the +evaluation of the function underlying the + operator. The steps for the +Vars would have their resvalue/resnull pointing directly to the appropriate +args[].value .isnull elements in the FunctionCallInfoBaseData struct that +is used by the function evaluation step, thus avoiding extra work to copy +the result values around. + +The last entry in a completed ExprState->steps array is always an +EEOP_DONE step; this removes the need to test for end-of-array while +iterating. Also, if the expression contains any variable references (to +user columns of the ExprContext's INNER, OUTER, or SCAN tuples), the steps +array begins with EEOP_*_FETCHSOME steps that ensure that the relevant +tuples have been deconstructed to make the required columns directly +available (cf. slot_getsomeattrs()). This allows individual Var-fetching +steps to be little more than an array lookup. + +Most of ExecInitExpr()'s work is done by the recursive function +ExecInitExprRec() and its subroutines. ExecInitExprRec() maps one Expr +node into the steps required for execution, recursing as needed for +sub-expressions. + +Each ExecInitExprRec() call has to specify where that subexpression's +results are to be stored (via the resv/resnull parameters). This allows +the above scenario of evaluating a (sub-)expression directly into +fcinfo->args[].value/isnull, but also requires some care: target Datum/isnull +variables may not be shared with another ExecInitExprRec() unless the +results are only needed by steps executing before further usages of those +target Datum/isnull variables. Due to the non-recursiveness of the +ExprEvalStep representation that's usually easy to guarantee. + +ExecInitExprRec() pushes new operations into the ExprState->steps array +using ExprEvalPushStep(). To keep the steps as a consecutively laid out +array, ExprEvalPushStep() has to repalloc the entire array when there's +not enough space. Because of that it is *not* allowed to point directly +into any of the steps during expression initialization. Therefore, the +resv/resnull for a subexpression usually point to some storage that is +palloc'd separately from the steps array. For instance, the +FunctionCallInfoBaseData for a function call step is separately allocated +rather than being part of the ExprEvalStep array. The overall result +of a complete expression is typically returned into the resvalue/resnull +fields of the ExprState node itself. + +Some steps, e.g. boolean expressions, allow skipping evaluation of +certain subexpressions. In the flat representation this amounts to +jumping to some later step rather than just continuing consecutively +with the next step. The target for such a jump is represented by +the integer index in the ExprState->steps array of the step to execute +next. (Compare the EEO_NEXT and EEO_JUMP macros in execExprInterp.c.) + +Typically, ExecInitExprRec() has to push a jumping step into the steps +array, then recursively generate steps for the subexpression that might +get skipped over, then go back and fix up the jump target index using +the now-known length of the subexpression's steps. This is handled by +adjust_jumps lists in execExpr.c. + +The last step in constructing an ExprState is to apply ExecReadyExpr(), +which readies it for execution using whichever execution method has been +selected. + + +Expression Evaluation +--------------------- + +To allow for different methods of expression evaluation, and for +better branch/jump target prediction, expressions are evaluated by +calling ExprState->evalfunc (via ExecEvalExpr() and friends). + +ExecReadyExpr() can choose the method of interpretation by setting +evalfunc to an appropriate function. The default execution function, +ExecInterpExpr, is implemented in execExprInterp.c; see its header +comment for details. Special-case evalfuncs are used for certain +especially-simple expressions. + +Note that a lot of the more complex expression evaluation steps, which are +less performance-critical than the simpler ones, are implemented as +separate functions outside the fast-path of expression execution, allowing +their implementation to be shared between interpreted and compiled +expression evaluation. This means that these helper functions are not +allowed to perform expression step dispatch themselves, as the method of +dispatch will vary based on the caller. The helpers therefore cannot call +for the execution of subexpressions; all subexpression results they need +must be computed by earlier steps. And dispatch to the following +expression step must be performed after returning from the helper. + + +Targetlist Evaluation +--------------------- + +ExecBuildProjectionInfo builds an ExprState that has the effect of +evaluating a targetlist into ExprState->resultslot. A generic targetlist +expression is executed by evaluating it as discussed above (storing the +result into the ExprState's resvalue/resnull fields) and then using an +EEOP_ASSIGN_TMP step to move the result into the appropriate tts_values[] +and tts_isnull[] array elements of the result slot. There are special +fast-path step types (EEOP_ASSIGN_*_VAR) to handle targetlist entries that +are simple Vars using only one step instead of two. + + +Memory Management +----------------- + +A "per query" memory context is created during CreateExecutorState(); +all storage allocated during an executor invocation is allocated in that +context or a child context. This allows easy reclamation of storage +during executor shutdown --- rather than messing with retail pfree's and +probable storage leaks, we just destroy the memory context. + +In particular, the plan state trees and expression state trees described +in the previous section are allocated in the per-query memory context. + +To avoid intra-query memory leaks, most processing while a query runs +is done in "per tuple" memory contexts, which are so-called because they +are typically reset to empty once per tuple. Per-tuple contexts are usually +associated with ExprContexts, and commonly each PlanState node has its own +ExprContext to evaluate its qual and targetlist expressions in. + + +Query Processing Control Flow +----------------------------- + +This is a sketch of control flow for full query processing: + + CreateQueryDesc + + ExecutorStart + CreateExecutorState + creates per-query context + switch to per-query context to run ExecInitNode + AfterTriggerBeginQuery + ExecInitNode --- recursively scans plan tree + ExecInitNode + recurse into subsidiary nodes + CreateExprContext + creates per-tuple context + ExecInitExpr + + ExecutorRun + ExecProcNode --- recursively called in per-query context + ExecEvalExpr --- called in per-tuple context + ResetExprContext --- to free memory + + ExecutorFinish + ExecPostprocessPlan --- run any unfinished ModifyTable nodes + AfterTriggerEndQuery + + ExecutorEnd + ExecEndNode --- recursively releases resources + FreeExecutorState + frees per-query context and child contexts + + FreeQueryDesc + +Per above comments, it's not really critical for ExecEndNode to free any +memory; it'll all go away in FreeExecutorState anyway. However, we do need to +be careful to close relations, drop buffer pins, etc, so we do need to scan +the plan state tree to find these sorts of resources. + + +The executor can also be used to evaluate simple expressions without any Plan +tree ("simple" meaning "no aggregates and no sub-selects", though such might +be hidden inside function calls). This case has a flow of control like + + CreateExecutorState + creates per-query context + + CreateExprContext -- or use GetPerTupleExprContext(estate) + creates per-tuple context + + ExecPrepareExpr + temporarily switch to per-query context + run the expression through expression_planner + ExecInitExpr + + Repeatedly do: + ExecEvalExprSwitchContext + ExecEvalExpr --- called in per-tuple context + ResetExprContext --- to free memory + + FreeExecutorState + frees per-query context, as well as ExprContext + (a separate FreeExprContext call is not necessary) + + +EvalPlanQual (READ COMMITTED Update Checking) +--------------------------------------------- + +For simple SELECTs, the executor need only pay attention to tuples that are +valid according to the snapshot seen by the current transaction (ie, they +were inserted by a previously committed transaction, and not deleted by any +previously committed transaction). However, for UPDATE and DELETE it is not +cool to modify or delete a tuple that's been modified by an open or +concurrently-committed transaction. If we are running in SERIALIZABLE +isolation level then we just raise an error when this condition is seen to +occur. In READ COMMITTED isolation level, we must work a lot harder. + +The basic idea in READ COMMITTED mode is to take the modified tuple +committed by the concurrent transaction (after waiting for it to commit, +if need be) and re-evaluate the query qualifications to see if it would +still meet the quals. If so, we regenerate the updated tuple (if we are +doing an UPDATE) from the modified tuple, and finally update/delete the +modified tuple. SELECT FOR UPDATE/SHARE behaves similarly, except that its +action is just to lock the modified tuple and return results based on that +version of the tuple. + +To implement this checking, we actually re-run the query from scratch for +each modified tuple (or set of tuples, for SELECT FOR UPDATE), with the +relation scan nodes tweaked to return only the current tuples --- either +the original ones, or the updated (and now locked) versions of the modified +tuple(s). If this query returns a tuple, then the modified tuple(s) pass +the quals (and the query output is the suitably modified update tuple, if +we're doing UPDATE). If no tuple is returned, then the modified tuple(s) +fail the quals, so we ignore the current result tuple and continue the +original query. + +In UPDATE/DELETE, only the target relation needs to be handled this way. +In SELECT FOR UPDATE, there may be multiple relations flagged FOR UPDATE, +so we obtain lock on the current tuple version in each such relation before +executing the recheck. + +It is also possible that there are relations in the query that are not +to be locked (they are neither the UPDATE/DELETE target nor specified to +be locked in SELECT FOR UPDATE/SHARE). When re-running the test query +we want to use the same rows from these relations that were joined to +the locked rows. For ordinary relations this can be implemented relatively +cheaply by including the row TID in the join outputs and re-fetching that +TID. (The re-fetch is expensive, but we're trying to optimize the normal +case where no re-test is needed.) We have also to consider non-table +relations, such as a ValuesScan or FunctionScan. For these, since there +is no equivalent of TID, the only practical solution seems to be to include +the entire row value in the join output row. + +We disallow set-returning functions in the targetlist of SELECT FOR UPDATE, +so as to ensure that at most one tuple can be returned for any particular +set of scan tuples. Otherwise we'd get duplicates due to the original +query returning the same set of scan tuples multiple times. Likewise, +SRFs are disallowed in an UPDATE's targetlist. There, they would have the +effect of the same row being updated multiple times, which is not very +useful --- and updates after the first would have no effect anyway. + + +Asynchronous Execution +---------------------- + +In cases where a node is waiting on an event external to the database system, +such as a ForeignScan awaiting network I/O, it's desirable for the node to +indicate that it cannot return any tuple immediately but may be able to do so +at a later time. A process which discovers this type of situation can always +handle it simply by blocking, but this may waste time that could be spent +executing some other part of the plan tree where progress could be made +immediately. This is particularly likely to occur when the plan tree contains +an Append node. Asynchronous execution runs multiple parts of an Append node +concurrently rather than serially to improve performance. + +For asynchronous execution, an Append node must first request a tuple from an +async-capable child node using ExecAsyncRequest. Next, it must execute the +asynchronous event loop using ExecAppendAsyncEventWait. Eventually, when a +child node to which an asynchronous request has been made produces a tuple, +the Append node will receive it from the event loop via ExecAsyncResponse. In +the current implementation of asynchronous execution, the only node type that +requests tuples from an async-capable child node is an Append, while the only +node type that might be async-capable is a ForeignScan. + +Typically, the ExecAsyncResponse callback is the only one required for nodes +that wish to request tuples asynchronously. On the other hand, async-capable +nodes generally need to implement three methods: + +1. When an asynchronous request is made, the node's ExecAsyncRequest callback + will be invoked; it should use ExecAsyncRequestPending to indicate that the + request is pending for a callback described below. Alternatively, it can + instead use ExecAsyncRequestDone if a result is available immediately. + +2. When the event loop wishes to wait or poll for file descriptor events, the + node's ExecAsyncConfigureWait callback will be invoked to configure the + file descriptor event for which the node wishes to wait. + +3. When the file descriptor becomes ready, the node's ExecAsyncNotify callback + will be invoked; like #1, it should use ExecAsyncRequestPending for another + callback or ExecAsyncRequestDone to return a result immediately. |