[FEATURE] Join query doesn't support index alias

[dai-chen]

Is your feature request related to a problem?

When using index alias instead of index name in JOIN query, NPE is thrown from SQL plugin.

What solution would you like?

JOIN query (or any SQL query) can handle index alias without issue.

Do you have any additional context?

Error stacktrace:

[2024-03-06T22:33:27,579][ERROR][o.o.s.l.p.RestSqlAction  ]  Client side error during query execution
java.lang.NullPointerException: Cannot invoke "org.opensearch.sql.legacy.esdomain.mapping.FieldMappings.has(String)" because "fieldMappings" is null
    at org.opensearch.sql.legacy.rewriter.matchtoterm.TermFieldRewriter.visit(TermFieldRewriter.java:129)
    at com.alibaba.druid.sql.ast.expr.SQLIdentifierExpr.accept0(SQLIdentifierExpr.java:65)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.expr.SQLBinaryOpExpr.accept0(SQLBinaryOpExpr.java:99)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock.accept0(MySqlSelectQueryBlock.java:258)
    at com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock.accept0(MySqlSelectQueryBlock.java:246)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.statement.SQLSelect.accept0(SQLSelect.java:85)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.expr.SQLQueryExpr.accept0(SQLQueryExpr.java:55)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at org.opensearch.sql.legacy.query.OpenSearchActionFactory.create(OpenSearchActionFactory.java:110)
    at org.opensearch.sql.legacy.plugin.SearchDao.explain(SearchDao.java:52)
    at org.opensearch.sql.legacy.plugin.RestSqlAction.explainRequest(RestSqlAction.java:208)
    at org.opensearch.sql.legacy.plugin.RestSqlAction.lambda$prepareRequest$1(RestSqlAction.java:160)
    at org.opensearch.sql.legacy.plugin.RestSQLQueryAction$1.onFailure(RestSQLQueryAction.java:130)
    at org.opensearch.sql.sql.SQLService.explain(SQLService.java:60)
    at org.opensearch.sql.legacy.plugin.RestSQLQueryAction.lambda$prepareRequest$2(RestSQLQueryAction.java:98)
    ...

[HabooshHaddad]

Hello
Any updates on this ?