add some comments and two missing function registrations

hibernate-core/src/main/java/org/hibernate/dialect/AbstractHANADialect.java

@@ -350,6 +350,7 @@ public abstract class AbstractHANADialect extends Dialect {
 		functionFactory.sinh();
 		functionFactory.tanh();
 		functionFactory.log10_log();
+		functionFactory.log();
 		functionFactory.bitand();
 		functionFactory.hourMinuteSecond();
 		functionFactory.yearMonthDay();

hibernate-core/src/main/java/org/hibernate/dialect/DerbyDialect.java

@@ -16,7 +16,6 @@ import org.hibernate.dialect.function.CaseLeastGreatestEmulation;
 import org.hibernate.dialect.function.InsertSubstringOverlayEmulation;
 import org.hibernate.dialect.identity.DB2IdentityColumnSupport;
 import org.hibernate.dialect.identity.IdentityColumnSupport;
-import org.hibernate.dialect.pagination.AbstractLimitHandler;
 import org.hibernate.dialect.pagination.DerbyLimitHandler;
 import org.hibernate.dialect.pagination.LimitHandler;
 import org.hibernate.dialect.sequence.DerbySequenceSupport;
@@ -51,7 +50,6 @@ import org.hibernate.sql.ast.spi.StandardSqlAstTranslatorFactory;
 import org.hibernate.sql.ast.tree.Statement;
 import org.hibernate.sql.exec.spi.JdbcOperation;
 import org.hibernate.tool.schema.extract.internal.SequenceInformationExtractorDerbyDatabaseImpl;
-import org.hibernate.tool.schema.extract.internal.SequenceInformationExtractorNoOpImpl;
 import org.hibernate.tool.schema.extract.spi.SequenceInformationExtractor;
 import org.hibernate.type.BasicType;
 import org.hibernate.type.BasicTypeRegistry;
@@ -60,7 +58,6 @@ import org.hibernate.type.StandardBasicTypes;
 import org.hibernate.type.descriptor.java.BigDecimalJavaType;
 import org.hibernate.type.descriptor.jdbc.DecimalJdbcType;
 import org.hibernate.type.descriptor.jdbc.ObjectNullResolvingJdbcType;
-import org.hibernate.type.descriptor.jdbc.SmallIntJdbcType;
 import org.hibernate.type.descriptor.jdbc.TimestampJdbcType;
 import org.hibernate.type.descriptor.jdbc.spi.JdbcTypeRegistry;
 import org.hibernate.type.descriptor.sql.internal.CapacityDependentDdlType;
@@ -254,6 +251,10 @@ public class DerbyDialect extends Dialect {
 		// AVG by default uses the input type, so we possibly need to cast the argument type, hence a special function
 		functionFactory.avg_castingNonDoubleArguments( this, SqlAstNodeRenderingMode.DEFAULT );
 
+		// Note that Derby does not have chr() / ascii() functions.
+		// It does have a function named char(), but it's really a
+		// sort of to_char() function.
+
 		functionFactory.concat_pipeOperator();
 		functionFactory.cot();
 		functionFactory.degrees();

hibernate-core/src/main/java/org/hibernate/dialect/MySQLDialect.java

@@ -46,6 +46,7 @@ import org.hibernate.query.sqm.IntervalType;
 import org.hibernate.query.sqm.NullOrdering;
 import org.hibernate.query.sqm.TemporalUnit;
 import org.hibernate.query.spi.QueryEngine;
+import org.hibernate.query.sqm.function.SqmFunctionRegistry;
 import org.hibernate.query.sqm.mutation.internal.temptable.AfterUseAction;
 import org.hibernate.dialect.temptable.TemporaryTable;
 import org.hibernate.query.sqm.mutation.internal.temptable.BeforeUseAction;
@@ -492,22 +493,28 @@ public class MySQLDialect extends Dialect {
 
 		BasicTypeRegistry basicTypeRegistry = queryEngine.getTypeConfiguration().getBasicTypeRegistry();
 
-		queryEngine.getSqmFunctionRegistry().noArgsBuilder( "localtime" )
+		SqmFunctionRegistry functionRegistry = queryEngine.getSqmFunctionRegistry();
+
+		functionRegistry.noArgsBuilder( "localtime" )
 				.setInvariantType(basicTypeRegistry.resolve( StandardBasicTypes.TIMESTAMP ))
 				.setUseParenthesesWhenNoArgs( false )
 				.register();
 
-		queryEngine.getSqmFunctionRegistry().patternDescriptorBuilder( "pi", "cast(pi() as double)" )
+		// pi() produces a value with 7 digits unless we're explicit
+		functionRegistry.patternDescriptorBuilder( "pi", "cast(pi() as double)" )
 				.setInvariantType(basicTypeRegistry.resolve( StandardBasicTypes.DOUBLE ))
 				.setExactArgumentCount(0)
 				.setArgumentListSignature("")
 				.register();
 
-		queryEngine.getSqmFunctionRegistry().patternDescriptorBuilder( "chr", "char(?1 using ascii)" )
+		// By default char() produces a binary string, not a character string.
+		// (Note also that char() is actually a variadic function in MySQL.)
+		functionRegistry.patternDescriptorBuilder( "chr", "char(?1 using ascii)" )
 				.setInvariantType(basicTypeRegistry.resolve( StandardBasicTypes.CHARACTER ))
 				.setExactArgumentCount(1)
 				.setParameterTypes(FunctionParameterType.INTEGER)
 				.register();
+		functionRegistry.registerAlternateKey( "char", "chr" );
 
 		// MySQL timestamp type defaults to precision 0 (seconds) but
 		// we want the standard default precision of 6 (microseconds)

hibernate-core/src/main/java/org/hibernate/dialect/function/CommonFunctionFactory.java

@@ -1696,6 +1696,12 @@ public class CommonFunctionFactory {
 				.register();
 	}
 
+	/**
+	 * Very widely supported, but we don't treat this as a "standard"
+	 * function because it's hard to emulate on any database that
+	 * doesn't have it (e.g. Derby) and because, well, ASCII. For the
+	 * same reason we don't consider chr()/char() as "standard".
+	 */
 	public void ascii() {
 		functionRegistry.namedDescriptorBuilder( "ascii" )
 				.setExactArgumentCount( 1 )