Use perfect hashing, instead of binary search, for keyword lookup.

We've been speculating for a long time that hash-based keyword lookup
ought to be faster than binary search, but up to now we hadn't found
a suitable tool for generating the hash function.  Joerg Sonnenberger
provided the inspiration, and sample code, to show us that rolling our
own generator wasn't a ridiculous idea.  Hence, do that.

The method used here requires a lookup table of approximately 4 bytes
per keyword, but that's less than what we saved in the predecessor commit
afb0d0712, so it's not a big problem.  The time savings is indeed
significant: preliminary testing suggests that the total time for raw
parsing (flex + bison phases) drops by ~20%.

Patch by me, but it owes its existence to Joerg Sonnenberger;
thanks also to John Naylor for review.

Discussion: https://postgr.es/m/20190103163340.GA15803@britannica.bec.de

src/common/Makefile

@@ -63,6 +63,11 @@ OBJS_FRONTEND = $(OBJS_COMMON) fe_memutils.o file_utils.o restricted_token.o
 OBJS_SHLIB = $(OBJS_FRONTEND:%.o=%_shlib.o)
 OBJS_SRV = $(OBJS_COMMON:%.o=%_srv.o)
 
+# where to find gen_keywordlist.pl and subsidiary files
+TOOLSDIR = $(top_srcdir)/src/tools
+GEN_KEYWORDLIST = $(PERL) -I $(TOOLSDIR) $(TOOLSDIR)/gen_keywordlist.pl
+GEN_KEYWORDLIST_DEPS = $(TOOLSDIR)/gen_keywordlist.pl $(TOOLSDIR)/PerfectHash.pm
+
 all: libpgcommon.a libpgcommon_shlib.a libpgcommon_srv.a
 
 distprep: kwlist_d.h
@@ -118,8 +123,8 @@ libpgcommon_srv.a: $(OBJS_SRV)
 	$(CC) $(CFLAGS) $(subst -DFRONTEND,, $(CPPFLAGS)) -c $< -o $@
 
 # generate SQL keyword lookup table to be included into keywords*.o.
-kwlist_d.h: $(top_srcdir)/src/include/parser/kwlist.h $(top_srcdir)/src/tools/gen_keywordlist.pl
+kwlist_d.h: $(top_srcdir)/src/include/parser/kwlist.h $(GEN_KEYWORDLIST_DEPS)
-	$(PERL) $(top_srcdir)/src/tools/gen_keywordlist.pl --extern $<
+	$(GEN_KEYWORDLIST) --extern $<
 
 # Dependencies of keywords*.o need to be managed explicitly to make sure
 # that you don't get broken parsing code, even in a non-enable-depend build.

src/common/kwlookup.c

@@ -35,60 +35,51 @@
  * receive a different case-normalization mapping.
  */
 int
-ScanKeywordLookup(const char *text,
+ScanKeywordLookup(const char *str,
 				  const ScanKeywordList *keywords)
 {
-	int			len,
+	size_t		len;
-				i;
+	int			h;
-	char		word[NAMEDATALEN];
+	const char *kw;
-	const char *kw_string;
-	const uint16 *kw_offsets;
-	const uint16 *low;
-	const uint16 *high;
-
-	len = strlen(text);
-
-	if (len > keywords->max_kw_len)
-		return -1;				/* too long to be any keyword */
-
-	/* We assume all keywords are shorter than NAMEDATALEN. */
-	Assert(len < NAMEDATALEN);
 
 	/*
-	 * Apply an ASCII-only downcasing.  We must not use tolower() since it may
+	 * Reject immediately if too long to be any keyword.  This saves useless
-	 * produce the wrong translation in some locales (eg, Turkish).
+	 * hashing and downcasing work on long strings.
 	 */
-	for (i = 0; i < len; i++)
+	len = strlen(str);
+	if (len > keywords->max_kw_len)
+		return -1;
+
+	/*
+	 * Compute the hash function.  We assume it was generated to produce
+	 * case-insensitive results.  Since it's a perfect hash, we need only
+	 * match to the specific keyword it identifies.
+	 */
+	h = keywords->hash(str, len);
+
+	/* An out-of-range result implies no match */
+	if (h < 0 || h >= keywords->num_keywords)
+		return -1;
+
+	/*
+	 * Compare character-by-character to see if we have a match, applying an
+	 * ASCII-only downcasing to the input characters.  We must not use
+	 * tolower() since it may produce the wrong translation in some locales
+	 * (eg, Turkish).
+	 */
+	kw = GetScanKeyword(h, keywords);
+	while (*str != '\0')
 	{
-		char		ch = text[i];
+		char		ch = *str++;
 
 		if (ch >= 'A' && ch <= 'Z')
 			ch += 'a' - 'A';
-		word[i] = ch;
+		if (ch != *kw++)
+			return -1;
 	}
-	word[len] = '\0';
+	if (*kw != '\0')
+		return -1;
 
-	/*
+	/* Success! */
-	 * Now do a binary search using plain strcmp() comparison.
+	return h;
-	 */
-	kw_string = keywords->kw_string;
-	kw_offsets = keywords->kw_offsets;
-	low = kw_offsets;
-	high = kw_offsets + (keywords->num_keywords - 1);
-	while (low <= high)
-	{
-		const uint16 *middle;
-		int			difference;
-
-		middle = low + (high - low) / 2;
-		difference = strcmp(kw_string + *middle, word);
-		if (difference == 0)
-			return middle - kw_offsets;
-		else if (difference < 0)
-			low = middle + 1;
-		else
-			high = middle - 1;
-	}
-
-	return -1;
 }

src/include/common/kwlookup.h

@@ -14,6 +14,9 @@
 #ifndef KWLOOKUP_H
 #define KWLOOKUP_H
 
+/* Hash function used by ScanKeywordLookup */
+typedef int (*ScanKeywordHashFunc) (const void *key, size_t keylen);
+
 /*
  * This struct contains the data needed by ScanKeywordLookup to perform a
  * search within a set of keywords.  The contents are typically generated by
@@ -23,6 +26,7 @@ typedef struct ScanKeywordList
 {
 	const char *kw_string;		/* all keywords in order, separated by \0 */
 	const uint16 *kw_offsets;	/* offsets to the start of each keyword */
+	ScanKeywordHashFunc hash;	/* perfect hash function for keywords */
 	int			num_keywords;	/* number of keywords */
 	int			max_kw_len;		/* length of longest keyword */
 } ScanKeywordList;

src/include/parser/kwlist.h

@@ -21,8 +21,7 @@
 /*
  * List of keyword (name, token-value, category) entries.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
- *		 search is used to locate entries.
  */
 
 /* name, value, category */

src/interfaces/ecpg/preproc/Makefile

@@ -28,7 +28,10 @@ OBJS=	preproc.o pgc.o type.o ecpg.o output.o parser.o \
 	keywords.o c_keywords.o ecpg_keywords.o typename.o descriptor.o variable.o \
 	$(WIN32RES)
 
-GEN_KEYWORDLIST = $(top_srcdir)/src/tools/gen_keywordlist.pl
+# where to find gen_keywordlist.pl and subsidiary files
+TOOLSDIR = $(top_srcdir)/src/tools
+GEN_KEYWORDLIST = $(PERL) -I $(TOOLSDIR) $(TOOLSDIR)/gen_keywordlist.pl
+GEN_KEYWORDLIST_DEPS = $(TOOLSDIR)/gen_keywordlist.pl $(TOOLSDIR)/PerfectHash.pm
 
 # Suppress parallel build to avoid a bug in GNU make 3.82
 # (see comments in ../Makefile)
@@ -56,11 +59,11 @@ preproc.y: ../../../backend/parser/gram.y parse.pl ecpg.addons ecpg.header ecpg.
 	$(PERL) $(srcdir)/check_rules.pl $(srcdir) $<
 
 # generate keyword headers
-c_kwlist_d.h: c_kwlist.h $(GEN_KEYWORDLIST)
+c_kwlist_d.h: c_kwlist.h $(GEN_KEYWORDLIST_DEPS)
-	$(PERL) $(GEN_KEYWORDLIST) --varname ScanCKeywords $<
+	$(GEN_KEYWORDLIST) --varname ScanCKeywords --no-case-fold $<
 
-ecpg_kwlist_d.h: ecpg_kwlist.h $(GEN_KEYWORDLIST)
+ecpg_kwlist_d.h: ecpg_kwlist.h $(GEN_KEYWORDLIST_DEPS)
-	$(PERL) $(GEN_KEYWORDLIST) --varname ScanECPGKeywords $<
+	$(GEN_KEYWORDLIST) --varname ScanECPGKeywords $<
 
 # Force these dependencies to be known even without dependency info built:
 ecpg_keywords.o c_keywords.o keywords.o preproc.o pgc.o parser.o: preproc.h

src/interfaces/ecpg/preproc/c_keywords.c

@@ -9,8 +9,6 @@
  */
 #include "postgres_fe.h"
 
-#include <ctype.h>
-
 #include "preproc_extern.h"
 #include "preproc.h"
 
@@ -32,39 +30,38 @@ static const uint16 ScanCKeywordTokens[] = {
  *
  * Returns the token value of the keyword, or -1 if no match.
  *
- * Do a binary search using plain strcmp() comparison.  This is much like
+ * Do a hash search using plain strcmp() comparison.  This is much like
  * ScanKeywordLookup(), except we want case-sensitive matching.
  */
 int
-ScanCKeywordLookup(const char *text)
+ScanCKeywordLookup(const char *str)
 {
-	const char *kw_string;
+	size_t		len;
-	const uint16 *kw_offsets;
+	int			h;
-	const uint16 *low;
+	const char *kw;
-	const uint16 *high;
 
-	if (strlen(text) > ScanCKeywords.max_kw_len)
+	/*
-		return -1;				/* too long to be any keyword */
+	 * Reject immediately if too long to be any keyword.  This saves useless
+	 * hashing work on long strings.
+	 */
+	len = strlen(str);
+	if (len > ScanCKeywords.max_kw_len)
+		return -1;
 
-	kw_string = ScanCKeywords.kw_string;
+	/*
-	kw_offsets = ScanCKeywords.kw_offsets;
+	 * Compute the hash function.  Since it's a perfect hash, we need only
-	low = kw_offsets;
+	 * match to the specific keyword it identifies.
-	high = kw_offsets + (ScanCKeywords.num_keywords - 1);
+	 */
+	h = ScanCKeywords_hash_func(str, len);
 
-	while (low <= high)
+	/* An out-of-range result implies no match */
-	{
+	if (h < 0 || h >= ScanCKeywords.num_keywords)
-		const uint16 *middle;
+		return -1;
-		int			difference;
 
-		middle = low + (high - low) / 2;
+	kw = GetScanKeyword(h, &ScanCKeywords);
-		difference = strcmp(kw_string + *middle, text);
+
-		if (difference == 0)
+	if (strcmp(kw, str) == 0)
-			return ScanCKeywordTokens[middle - kw_offsets];
+		return ScanCKeywordTokens[h];
-		else if (difference < 0)
-			low = middle + 1;
-		else
-			high = middle - 1;
-	}
 
 	return -1;
 }

src/interfaces/ecpg/preproc/c_kwlist.h

@@ -20,8 +20,7 @@
 /*
  * List of (keyword-name, keyword-token-value) pairs.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
- *		 search is used to locate entries.
  */
 
 /* name, value */

src/interfaces/ecpg/preproc/ecpg_kwlist.h

@@ -20,8 +20,7 @@
 /*
  * List of (keyword-name, keyword-token-value) pairs.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
- *		 search is used to locate entries.
  */
 
 /* name, value */

src/pl/plpgsql/src/Makefile

@@ -29,7 +29,10 @@ REGRESS_OPTS = --dbname=$(PL_TESTDB)
 REGRESS = plpgsql_call plpgsql_control plpgsql_domain plpgsql_record \
 	plpgsql_cache plpgsql_transaction plpgsql_trigger plpgsql_varprops
 
-GEN_KEYWORDLIST = $(top_srcdir)/src/tools/gen_keywordlist.pl
+# where to find gen_keywordlist.pl and subsidiary files
+TOOLSDIR = $(top_srcdir)/src/tools
+GEN_KEYWORDLIST = $(PERL) -I $(TOOLSDIR) $(TOOLSDIR)/gen_keywordlist.pl
+GEN_KEYWORDLIST_DEPS = $(TOOLSDIR)/gen_keywordlist.pl $(TOOLSDIR)/PerfectHash.pm
 
 all: all-lib
 
@@ -76,11 +79,11 @@ plerrcodes.h: $(top_srcdir)/src/backend/utils/errcodes.txt generate-plerrcodes.p
 	$(PERL) $(srcdir)/generate-plerrcodes.pl $< > $@
 
 # generate keyword headers for the scanner
-pl_reserved_kwlist_d.h: pl_reserved_kwlist.h $(GEN_KEYWORDLIST)
+pl_reserved_kwlist_d.h: pl_reserved_kwlist.h $(GEN_KEYWORDLIST_DEPS)
-	$(PERL) $(GEN_KEYWORDLIST) --varname ReservedPLKeywords $<
+	$(GEN_KEYWORDLIST) --varname ReservedPLKeywords $<
 
-pl_unreserved_kwlist_d.h: pl_unreserved_kwlist.h $(GEN_KEYWORDLIST)
+pl_unreserved_kwlist_d.h: pl_unreserved_kwlist.h $(GEN_KEYWORDLIST_DEPS)
-	$(PERL) $(GEN_KEYWORDLIST) --varname UnreservedPLKeywords $<
+	$(GEN_KEYWORDLIST) --varname UnreservedPLKeywords $<
 
 
 check: submake

src/pl/plpgsql/src/pl_reserved_kwlist.h

@@ -20,10 +20,9 @@
 /*
  * List of (keyword-name, keyword-token-value) pairs.
  *
- * Be careful not to put the same word in both lists.
+ * Be careful not to put the same word into pl_unreserved_kwlist.h.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
- *		 search is used to locate entries.
  */
 
 /* name, value */

src/pl/plpgsql/src/pl_unreserved_kwlist.h

@@ -20,11 +20,10 @@
 /*
  * List of (keyword-name, keyword-token-value) pairs.
  *
- * Be careful not to put the same word in both lists.  Also be sure that
+ * Be careful not to put the same word into pl_reserved_kwlist.h.  Also be
- * pl_gram.y's unreserved_keyword production agrees with this list.
+ * sure that pl_gram.y's unreserved_keyword production agrees with this list.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
- *		 search is used to locate entries.
  */
 
 /* name, value */

src/tools/PerfectHash.pm

@@ -0,0 +1,376 @@
+#----------------------------------------------------------------------
+#
+# PerfectHash.pm
+#    Perl module that constructs minimal perfect hash functions
+#
+# This code constructs a minimal perfect hash function for the given
+# set of keys, using an algorithm described in
+# "An optimal algorithm for generating minimal perfect hash functions"
+# by Czech, Havas and Majewski in Information Processing Letters,
+# 43(5):256-264, October 1992.
+# This implementation is loosely based on NetBSD's "nbperf",
+# which was written by Joerg Sonnenberger.
+#
+# The resulting hash function is perfect in the sense that if the presented
+# key is one of the original set, it will return the key's index in the set
+# (in range 0..N-1).  However, the caller must still verify the match,
+# as false positives are possible.  Also, the hash function may return
+# values that are out of range (negative or >= N), due to summing unrelated
+# hashtable entries.  This indicates that the presented key is definitely
+# not in the set.
+#
+#
+# Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+# Portions Copyright (c) 1994, Regents of the University of California
+#
+# src/tools/PerfectHash.pm
+#
+#----------------------------------------------------------------------
+
+package PerfectHash;
+
+use strict;
+use warnings;
+
+
+# At runtime, we'll compute two simple hash functions of the input key,
+# and use them to index into a mapping table.  The hash functions are just
+# multiply-and-add in uint32 arithmetic, with different multipliers and
+# initial seeds.  All the complexity in this module is concerned with
+# selecting hash parameters that will work and building the mapping table.
+
+# We support making case-insensitive hash functions, though this only
+# works for a strict-ASCII interpretation of case insensitivity,
+# ie, A-Z maps onto a-z and nothing else.
+my $case_fold = 0;
+
+
+#
+# Construct a C function implementing a perfect hash for the given keys.
+# The C function definition is returned as a string.
+#
+# The keys should be passed as an array reference.  They can be any set
+# of Perl strings; it is caller's responsibility that there not be any
+# duplicates.  (Note that the "strings" can be binary data, but hashing
+# e.g. OIDs has endianness hazards that callers must overcome.)
+#
+# The name to use for the function is specified as the second argument.
+# It will be a global function by default, but the caller may prepend
+# "static " to the result string if it wants a static function.
+#
+# Additional options can be specified as keyword-style arguments:
+#
+# case_fold => bool
+# If specified as true, the hash function is case-insensitive, for the
+# limited idea of case-insensitivity explained above.
+#
+# fixed_key_length => N
+# If specified, all keys are assumed to have length N bytes, and the
+# hash function signature will be just "int f(const void *key)"
+# rather than "int f(const void *key, size_t keylen)".
+#
+sub generate_hash_function
+{
+	my ($keys_ref, $funcname, %options) = @_;
+
+	# It's not worth passing this around as a parameter; just use a global.
+	$case_fold = $options{case_fold} || 0;
+
+	# Try different hash function parameters until we find a set that works
+	# for these keys.  The multipliers are chosen to be primes that are cheap
+	# to calculate via shift-and-add, so don't change them without care.
+	# (Commonly, random seeds are tried, but we want reproducible results
+	# from this program so we don't do that.)
+	my $hash_mult1 = 31;
+	my $hash_mult2;
+	my $hash_seed1;
+	my $hash_seed2;
+	my @subresult;
+  FIND_PARAMS:
+	foreach (127, 257, 521, 1033, 2053)
+	{
+		$hash_mult2 = $_;    # "foreach $hash_mult2" doesn't work
+		for ($hash_seed1 = 0; $hash_seed1 < 10; $hash_seed1++)
+		{
+			for ($hash_seed2 = 0; $hash_seed2 < 10; $hash_seed2++)
+			{
+				@subresult = _construct_hash_table(
+					$keys_ref,   $hash_mult1, $hash_mult2,
+					$hash_seed1, $hash_seed2);
+				last FIND_PARAMS if @subresult;
+			}
+		}
+	}
+
+	# Choke if we couldn't find a workable set of parameters.
+	die "failed to generate perfect hash" if !@subresult;
+
+	# Extract info from _construct_hash_table's result array.
+	my $elemtype = $subresult[0];
+	my @hashtab  = @{ $subresult[1] };
+	my $nhash    = scalar(@hashtab);
+
+	# OK, construct the hash function definition including the hash table.
+	my $f = '';
+	$f .= sprintf "int\n";
+	if (defined $options{fixed_key_length})
+	{
+		$f .= sprintf "%s(const void *key)\n{\n", $funcname;
+	}
+	else
+	{
+		$f .= sprintf "%s(const void *key, size_t keylen)\n{\n", $funcname;
+	}
+	$f .= sprintf "\tstatic const %s h[%d] = {\n", $elemtype, $nhash;
+	for (my $i = 0; $i < $nhash; $i++)
+	{
+		$f .= sprintf "%s%6d,%s",
+		  ($i % 8 == 0 ? "\t\t" : " "),
+		  $hashtab[$i],
+		  ($i % 8 == 7 ? "\n" : "");
+	}
+	$f .= sprintf "\n" if ($nhash % 8 != 0);
+	$f .= sprintf "\t};\n\n";
+	$f .= sprintf "\tconst unsigned char *k = key;\n";
+	$f .= sprintf "\tsize_t\t\tkeylen = %d;\n", $options{fixed_key_length}
+	  if (defined $options{fixed_key_length});
+	$f .= sprintf "\tuint32\t\ta = %d;\n",   $hash_seed1;
+	$f .= sprintf "\tuint32\t\tb = %d;\n\n", $hash_seed2;
+	$f .= sprintf "\twhile (keylen--)\n\t{\n";
+	$f .= sprintf "\t\tunsigned char c = *k++";
+	$f .= sprintf " | 0x20" if $case_fold;    # see comment below
+	$f .= sprintf ";\n\n";
+	$f .= sprintf "\t\ta = a * %d + c;\n", $hash_mult1;
+	$f .= sprintf "\t\tb = b * %d + c;\n", $hash_mult2;
+	$f .= sprintf "\t}\n";
+	$f .= sprintf "\treturn h[a %% %d] + h[b %% %d];\n", $nhash, $nhash;
+	$f .= sprintf "}\n";
+
+	return $f;
+}
+
+
+# Calculate a hash function as the run-time code will do.
+#
+# If we are making a case-insensitive hash function, we implement that
+# by OR'ing 0x20 into each byte of the key.  This correctly transforms
+# upper-case ASCII into lower-case ASCII, while not changing digits or
+# dollar signs.  (It does change '_', as well as other characters not
+# likely to appear in keywords; this has little effect on the hash's
+# ability to discriminate keywords.)
+sub _calc_hash
+{
+	my ($key, $mult, $seed) = @_;
+
+	my $result = $seed;
+	for my $c (split //, $key)
+	{
+		my $cn = ord($c);
+		$cn |= 0x20 if $case_fold;
+		$result = ($result * $mult + $cn) % 4294967296;
+	}
+	return $result;
+}
+
+
+# Attempt to construct a mapping table for a minimal perfect hash function
+# for the given keys, using the specified hash parameters.
+#
+# Returns an array containing the mapping table element type name as the
+# first element, and a ref to an array of the table values as the second.
+#
+# Returns an empty array on failure; then caller should choose different
+# hash parameter(s) and try again.
+sub _construct_hash_table
+{
+	my ($keys_ref, $hash_mult1, $hash_mult2, $hash_seed1, $hash_seed2) = @_;
+	my @keys = @{$keys_ref};
+
+	# This algorithm is based on a graph whose edges correspond to the
+	# keys and whose vertices correspond to entries of the mapping table.
+	# A key's edge links the two vertices whose indexes are the outputs of
+	# the two hash functions for that key.  For K keys, the mapping
+	# table must have at least 2*K+1 entries, guaranteeing that there's at
+	# least one unused entry.  (In principle, larger mapping tables make it
+	# easier to find a workable hash and increase the number of inputs that
+	# can be rejected due to touching unused hashtable entries.  In practice,
+	# neither effect seems strong enough to justify using a larger table.)
+	my $nedges = scalar @keys;       # number of edges
+	my $nverts = 2 * $nedges + 1;    # number of vertices
+
+	# However, it would be very bad if $nverts were exactly equal to either
+	# $hash_mult1 or $hash_mult2: effectively, that hash function would be
+	# sensitive to only the last byte of each key.  Cases where $nverts is a
+	# multiple of either multiplier likewise lose information.  (But $nverts
+	# can't actually divide them, if they've been intelligently chosen as
+	# primes.)  We can avoid such problems by adjusting the table size.
+	while ($nverts % $hash_mult1 == 0
+		|| $nverts % $hash_mult2 == 0)
+	{
+		$nverts++;
+	}
+
+	# Initialize the array of edges.
+	my @E = ();
+	foreach my $kw (@keys)
+	{
+		# Calculate hashes for this key.
+		# The hashes are immediately reduced modulo the mapping table size.
+		my $hash1 = _calc_hash($kw, $hash_mult1, $hash_seed1) % $nverts;
+		my $hash2 = _calc_hash($kw, $hash_mult2, $hash_seed2) % $nverts;
+
+		# If the two hashes are the same for any key, we have to fail
+		# since this edge would itself form a cycle in the graph.
+		return () if $hash1 == $hash2;
+
+		# Add the edge for this key.
+		push @E, { left => $hash1, right => $hash2 };
+	}
+
+	# Initialize the array of vertices, giving them all empty lists
+	# of associated edges.  (The lists will be hashes of edge numbers.)
+	my @V = ();
+	for (my $v = 0; $v < $nverts; $v++)
+	{
+		push @V, { edges => {} };
+	}
+
+	# Insert each edge in the lists of edges connected to its vertices.
+	for (my $e = 0; $e < $nedges; $e++)
+	{
+		my $v = $E[$e]{left};
+		$V[$v]{edges}->{$e} = 1;
+
+		$v = $E[$e]{right};
+		$V[$v]{edges}->{$e} = 1;
+	}
+
+	# Now we attempt to prove the graph acyclic.
+	# A cycle-free graph is either empty or has some vertex of degree 1.
+	# Removing the edge attached to that vertex doesn't change this property,
+	# so doing that repeatedly will reduce the size of the graph.
+	# If the graph is empty at the end of the process, it was acyclic.
+	# We track the order of edge removal so that the next phase can process
+	# them in reverse order of removal.
+	my @output_order = ();
+
+	# Consider each vertex as a possible starting point for edge-removal.
+	for (my $startv = 0; $startv < $nverts; $startv++)
+	{
+		my $v = $startv;
+
+		# If vertex v is of degree 1 (i.e. exactly 1 edge connects to it),
+		# remove that edge, and then consider the edge's other vertex to see
+		# if it is now of degree 1.  The inner loop repeats until reaching a
+		# vertex not of degree 1.
+		while (scalar(keys(%{ $V[$v]{edges} })) == 1)
+		{
+			# Unlink its only edge.
+			my $e = (keys(%{ $V[$v]{edges} }))[0];
+			delete($V[$v]{edges}->{$e});
+
+			# Unlink the edge from its other vertex, too.
+			my $v2 = $E[$e]{left};
+			$v2 = $E[$e]{right} if ($v2 == $v);
+			delete($V[$v2]{edges}->{$e});
+
+			# Push e onto the front of the output-order list.
+			unshift @output_order, $e;
+
+			# Consider v2 on next iteration of inner loop.
+			$v = $v2;
+		}
+	}
+
+	# We succeeded only if all edges were removed from the graph.
+	return () if (scalar(@output_order) != $nedges);
+
+	# OK, build the hash table of size $nverts.
+	my @hashtab = (0) x $nverts;
+	# We need a "visited" flag array in this step, too.
+	my @visited = (0) x $nverts;
+
+	# The goal is that for any key, the sum of the hash table entries for
+	# its first and second hash values is the desired output (i.e., the key
+	# number).  By assigning hash table values in the selected edge order,
+	# we can guarantee that that's true.  This works because the edge first
+	# removed from the graph (and hence last to be visited here) must have
+	# at least one vertex it shared with no other edge; hence it will have at
+	# least one vertex (hashtable entry) still unvisited when we reach it here,
+	# and we can assign that unvisited entry a value that makes the sum come
+	# out as we wish.  By induction, the same holds for all the other edges.
+	foreach my $e (@output_order)
+	{
+		my $l = $E[$e]{left};
+		my $r = $E[$e]{right};
+		if (!$visited[$l])
+		{
+			# $hashtab[$r] might be zero, or some previously assigned value.
+			$hashtab[$l] = $e - $hashtab[$r];
+		}
+		else
+		{
+			die "oops, doubly used hashtab entry" if $visited[$r];
+			# $hashtab[$l] might be zero, or some previously assigned value.
+			$hashtab[$r] = $e - $hashtab[$l];
+		}
+		# Now freeze both of these hashtab entries.
+		$visited[$l] = 1;
+		$visited[$r] = 1;
+	}
+
+	# Detect range of values needed in hash table.
+	my $hmin = $nedges;
+	my $hmax = 0;
+	for (my $v = 0; $v < $nverts; $v++)
+	{
+		$hmin = $hashtab[$v] if $hashtab[$v] < $hmin;
+		$hmax = $hashtab[$v] if $hashtab[$v] > $hmax;
+	}
+
+	# Choose width of hashtable entries.  In addition to the actual values,
+	# we need to be able to store a flag for unused entries, and we wish to
+	# have the property that adding any other entry value to the flag gives
+	# an out-of-range result (>= $nedges).
+	my $elemtype;
+	my $unused_flag;
+
+	if (   $hmin >= -0x7F
+		&& $hmax <= 0x7F
+		&& $hmin + 0x7F >= $nedges)
+	{
+		# int8 will work
+		$elemtype    = 'int8';
+		$unused_flag = 0x7F;
+	}
+	elsif ($hmin >= -0x7FFF
+		&& $hmax <= 0x7FFF
+		&& $hmin + 0x7FFF >= $nedges)
+	{
+		# int16 will work
+		$elemtype    = 'int16';
+		$unused_flag = 0x7FFF;
+	}
+	elsif ($hmin >= -0x7FFFFFFF
+		&& $hmax <= 0x7FFFFFFF
+		&& $hmin + 0x3FFFFFFF >= $nedges)
+	{
+		# int32 will work
+		$elemtype    = 'int32';
+		$unused_flag = 0x3FFFFFFF;
+	}
+	else
+	{
+		die "hash table values too wide";
+	}
+
+	# Set any unvisited hashtable entries to $unused_flag.
+	for (my $v = 0; $v < $nverts; $v++)
+	{
+		$hashtab[$v] = $unused_flag if !$visited[$v];
+	}
+
+	return ($elemtype, \@hashtab);
+}
+
+1;

src/tools/gen_keywordlist.pl

@@ -14,6 +14,12 @@
 # variable named according to the -v switch ("ScanKeywords" by default).
 # The variable is marked "static" unless the -e switch is given.
 #
+# ScanKeywordList uses hash-based lookup, so this script also selects
+# a minimal perfect hash function for the keyword set, and emits a
+# static hash function that is referenced in the ScanKeywordList struct.
+# The hash function is case-insensitive unless --no-case-fold is specified.
+# Note that case folding works correctly only for all-ASCII keywords!
+#
 #
 # Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 # Portions Copyright (c) 1994, Regents of the University of California
@@ -25,15 +31,18 @@
 use strict;
 use warnings;
 use Getopt::Long;
+use PerfectHash;
 
 my $output_path = '';
 my $extern = 0;
+my $case_fold = 1;
 my $varname = 'ScanKeywords';
 
 GetOptions(
-	'output:s' => \$output_path,
+	'output:s'   => \$output_path,
-	'extern'   => \$extern,
+	'extern'     => \$extern,
-	'varname:s' => \$varname) || usage();
+	'case-fold!' => \$case_fold,
+	'varname:s'  => \$varname) || usage();
 
 my $kw_input_file = shift @ARGV || die "No input file.\n";
 
@@ -87,7 +96,22 @@ while (<$kif>)
 	}
 }
 
+# When being case-insensitive, insist that the input be all-lower-case.
+if ($case_fold)
+{
+	foreach my $kw (@keywords)
+	{
+		die qq|The keyword "$kw" is not lower-case in $kw_input_file\n|
+		  if ($kw ne lc $kw);
+	}
+}
+
 # Error out if the keyword names are not in ASCII order.
+#
+# While this isn't really necessary with hash-based lookup, it's still
+# helpful because it provides a cheap way to reject duplicate keywords.
+# Also, insisting on sorted order ensures that code that scans the keyword
+# table linearly will see the keywords in a canonical order.
 for my $i (0..$#keywords - 1)
 {
 	die qq|The keyword "$keywords[$i + 1]" is out of order in $kw_input_file\n|
@@ -128,15 +152,25 @@ print $kwdef "};\n\n";
 
 printf $kwdef "#define %s_NUM_KEYWORDS %d\n\n", uc $varname, scalar @keywords;
 
+# Emit the definition of the hash function.
+
+my $funcname = $varname . "_hash_func";
+
+my $f = PerfectHash::generate_hash_function(\@keywords, $funcname,
+	case_fold => $case_fold);
+
+printf $kwdef qq|static %s\n|, $f;
+
 # Emit the struct that wraps all this lookup info into one variable.
 
-print $kwdef "static " if !$extern;
+printf $kwdef "static " if !$extern;
 printf $kwdef "const ScanKeywordList %s = {\n", $varname;
 printf $kwdef qq|\t%s_kw_string,\n|, $varname;
 printf $kwdef qq|\t%s_kw_offsets,\n|, $varname;
+printf $kwdef qq|\t%s,\n|, $funcname;
 printf $kwdef qq|\t%s_NUM_KEYWORDS,\n|, uc $varname;
 printf $kwdef qq|\t%d\n|, $max_len;
-print $kwdef "};\n\n";
+printf $kwdef "};\n\n";
 
 printf $kwdef "#endif\t\t\t\t\t\t\t/* %s_H */\n", uc $base_filename;
 
@@ -144,10 +178,11 @@ printf $kwdef "#endif\t\t\t\t\t\t\t/* %s_H */\n", uc $base_filename;
 sub usage
 {
 	die <<EOM;
-Usage: gen_keywordlist.pl [--output/-o <path>] [--varname/-v <varname>] [--extern/-e] input_file
+Usage: gen_keywordlist.pl [--output/-o <path>] [--varname/-v <varname>] [--extern/-e] [--[no-]case-fold] input_file
-    --output   Output directory (default '.')
+    --output        Output directory (default '.')
-    --varname  Name for ScanKeywordList variable (default 'ScanKeywords')
+    --varname       Name for ScanKeywordList variable (default 'ScanKeywords')
-    --extern   Allow the ScanKeywordList variable to be globally visible
+    --extern        Allow the ScanKeywordList variable to be globally visible
+    --no-case-fold  Keyword matching is to be case-sensitive
 
 gen_keywordlist.pl transforms a list of keywords into a ScanKeywordList.
 The output filename is derived from the input file by inserting _d,

src/tools/msvc/Solution.pm

@@ -414,7 +414,7 @@ sub GenerateFiles
 			'src/include/parser/kwlist.h'))
 	{
 		print "Generating kwlist_d.h...\n";
-		system('perl src/tools/gen_keywordlist.pl --extern -o src/common src/include/parser/kwlist.h');
+		system('perl -I src/tools src/tools/gen_keywordlist.pl --extern -o src/common src/include/parser/kwlist.h');
 	}
 
 	if (IsNewer(
@@ -426,8 +426,8 @@ sub GenerateFiles
 	{
 		print "Generating pl_reserved_kwlist_d.h and pl_unreserved_kwlist_d.h...\n";
 		chdir('src/pl/plpgsql/src');
-		system('perl ../../../tools/gen_keywordlist.pl --varname ReservedPLKeywords pl_reserved_kwlist.h');
+		system('perl -I ../../../tools ../../../tools/gen_keywordlist.pl --varname ReservedPLKeywords pl_reserved_kwlist.h');
-		system('perl ../../../tools/gen_keywordlist.pl --varname UnreservedPLKeywords pl_unreserved_kwlist.h');
+		system('perl -I ../../../tools ../../../tools/gen_keywordlist.pl --varname UnreservedPLKeywords pl_unreserved_kwlist.h');
 		chdir('../../../..');
 	}
 
@@ -440,8 +440,8 @@ sub GenerateFiles
 	{
 		print "Generating c_kwlist_d.h and ecpg_kwlist_d.h...\n";
 		chdir('src/interfaces/ecpg/preproc');
-		system('perl ../../../tools/gen_keywordlist.pl --varname ScanCKeywords c_kwlist.h');
+		system('perl -I ../../../tools ../../../tools/gen_keywordlist.pl --varname ScanCKeywords --no-case-fold c_kwlist.h');
-		system('perl ../../../tools/gen_keywordlist.pl --varname ScanECPGKeywords ecpg_kwlist.h');
+		system('perl -I ../../../tools ../../../tools/gen_keywordlist.pl --varname ScanECPGKeywords ecpg_kwlist.h');
 		chdir('../../../..');
 	}