Faster expression evaluation and targetlist projection.

This replaces the old, recursive tree-walk based evaluation, with
non-recursive, opcode dispatch based, expression evaluation.
Projection is now implemented as part of expression evaluation.

This both leads to significant performance improvements, and makes
future just-in-time compilation of expressions easier.

The speed gains primarily come from:
- non-recursive implementation reduces stack usage / overhead
- simple sub-expressions are implemented with a single jump, without
  function calls
- sharing some state between different sub-expressions
- reduced amount of indirect/hard to predict memory accesses by laying
  out operation metadata sequentially; including the avoidance of
  nearly all of the previously used linked lists
- more code has been moved to expression initialization, avoiding
  constant re-checks at evaluation time

Future just-in-time compilation (JIT) has become easier, as
demonstrated by released patches intended to be merged in a later
release, for primarily two reasons: Firstly, due to a stricter split
between expression initialization and evaluation, less code has to be
handled by the JIT. Secondly, due to the non-recursive nature of the
generated "instructions", less performance-critical code-paths can
easily be shared between interpreted and compiled evaluation.

The new framework allows for significant future optimizations. E.g.:
- basic infrastructure for to later reduce the per executor-startup
  overhead of expression evaluation, by caching state in prepared
  statements.  That'd be helpful in OLTPish scenarios where
  initialization overhead is measurable.
- optimizing the generated "code". A number of proposals for potential
  work has already been made.
- optimizing the interpreter. Similarly a number of proposals have
  been made here too.

The move of logic into the expression initialization step leads to some
backward-incompatible changes:
- Function permission checks are now done during expression
  initialization, whereas previously they were done during
  execution. In edge cases this can lead to errors being raised that
  previously wouldn't have been, e.g. a NULL array being coerced to a
  different array type previously didn't perform checks.
- The set of domain constraints to be checked, is now evaluated once
  during expression initialization, previously it was re-built
  every time a domain check was evaluated. For normal queries this
  doesn't change much, but e.g. for plpgsql functions, which caches
  ExprStates, the old set could stick around longer.  The behavior
  around might still change.

Author: Andres Freund, with significant changes by Tom Lane,
	changes by Heikki Linnakangas
Reviewed-By: Tom Lane, Heikki Linnakangas
Discussion: https://postgr.es/m/20161206034955.bh33paeralxbtluv@alap3.anarazel.de

1 files changed, 163 insertions, 13 deletions

diff --git a/src/backend/executor/README b/src/backend/executor/README
index f1d1e4c76ce..a0045067fb8 100644
--- a/src/backend/executor/README
+++ b/src/backend/executor/README
@@ -44,21 +44,171 @@ Plan Trees and State Trees
 --------------------------
 
 The plan tree delivered by the planner contains a tree of Plan nodes (struct
-types derived from struct Plan).  Each Plan node may have expression trees
-associated with it, to represent its target list, qualification conditions,
-etc.  During executor startup we build a parallel tree of identical structure
-containing executor state nodes --- every plan and expression 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 as 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.
+types derived from struct Plan).  During executor startup we build a parallel
+tree of identical structure containing executor state nodes --- 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.
+
+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 their corresponding
-ExprState nodes.  (Actually, there are also List nodes, which are used as
-"glue" in all four kinds of tree.)
+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
+arg[] and argnull[] array elements in the FunctionCallInfoData 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->arg/argnull, 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
+FunctionCallInfoData 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 ExprEvalExpr() and friends).
+
+ExprReadyExpr() 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