deparse_no_escape_string_syntax.patch

Spell string constants the way Postgres does, without E'' escape syntax.

deparseStringLiteral wraps any value containing a backslash in E'' and
doubles the backslashes. The comment explains why: it is copied from
postgres_fdw/deparse.c, which ships SQL to a remote server whose
standard_conforming_strings it cannot see, so it picks the spelling that
is safe under either setting.

A general-purpose deparser has no remote server. Postgres' own
parse-tree-to-SQL path, simple_quote_literal() in ruleutils.c, does the
opposite and says so:

    We form the string literal according to the prevailing setting of
    standard_conforming_strings; we never use E''.

The difference is observable. pg_get_constraintdef() on
CHECK (c ~ '^\d+$') returns the plain literal, while deparsing the same
parse tree here returns E'^\\d+$'. Ordinary statements therefore fail the
textual round-trip this repo's deparse tests are built on -- among them
SELECT regexp_replace(x, '\s+', ' '), which is unremarkable SQL.

E'' is also a Postgres extension rather than standard SQL, so it does not
survive being handed to other engines. We deparse Postgres parse trees and
send the result to ClickHouse, which has no E prefix: it lexes the E as an
identifier and rejects the whole query. Postgres made this same change for
the same reason in 2006, so that pg_dump output could load into other
databases without the backslash doubling.

Keying the doubling off standard_conforming_strings rather than hardcoding
it also honours PG_QUERY_DISABLE_STANDARD_CONFORMING_STRINGS, which this
library already exposes and which the deparser previously ignored.

Upstreaming this needs one expectation updated: deparse_tests.c pins
CREATE DOMAIN us_postal_code with E'^\\d{5}$' inputs, copied from the
Postgres docs, which now round-trip as plain literals. We do not run that
suite here, so the patch is left minimal.

diff --git a/src/postgres_deparse.c b/src/postgres_deparse.c
index 58401ad..a018112 100644
--- a/src/postgres_deparse.c
+++ b/src/postgres_deparse.c
@@ -13,6 +13,7 @@
 #include "nodes/nodes.h"
 #include "nodes/parsenodes.h"
 #include "nodes/pg_list.h"
+#include "parser/parser.h"
 #include "utils/builtins.h"
 #include "utils/datetime.h"
 #include "utils/timestamp.h"
@@ -675,19 +676,17 @@ deparseStringLiteral(DeparseState *state, const char *val)
 	const char *valptr;
 
 	/*
-	 * Rather than making assumptions about the remote server's value of
-	 * standard_conforming_strings, always use E'foo' syntax if there are any
-	 * backslashes.  This will fail on remote servers before 8.1, but those
-	 * are long out of support.
+	 * We form the string literal according to the prevailing setting of
+	 * standard_conforming_strings; we never use E''.  This matches
+	 * simple_quote_literal() in ruleutils.c, which is what Postgres itself
+	 * uses to turn a parse tree back into SQL.
 	 */
-	if (strchr(val, '\\') != NULL)
-		deparseAppendStringInfoChar(state, ESCAPE_STRING_SYNTAX);
 	deparseAppendStringInfoChar(state, '\'');
 	for (valptr = val; *valptr; valptr++)
 	{
 		char		ch = *valptr;
 
-		if (SQL_STR_DOUBLE(ch, true))
+		if (SQL_STR_DOUBLE(ch, !standard_conforming_strings))
 			deparseAppendStringInfoChar(state, ch);
 		deparseAppendStringInfoChar(state, ch);
 	}