Add describe and show covering index SQL support

[dai-chen]

Description

1. Add DESC INDEX statement support
2. Add SHOW INDEX index statement support based on a new getAllIndexMetadata() API in Flint client

Example

spark-sql> SHOW INDEX ON stream.lineitem_tiny;

spark-sql> CREATE INDEX orderkey_and_quantity
         > ON stream.lineitem_tiny (l_orderkey, l_quantity)
         > WITH (auto_refresh = true);

spark-sql> SHOW INDEX ON stream.lineitem_tiny;
index_name
orderkey_and_quantity

spark-sql> CREATE INDEX discount_and_quantity
         > ON stream.lineitem_tiny (l_discount, l_quantity)
         > WITH (auto_refresh = true);

spark-sql> SHOW INDEX ON stream.lineitem_tiny;
index_name
discount_and_quantity
orderkey_and_quantity

spark-sql> DESC INDEX discount_and_quantity ON stream.lineitem_tiny;
indexed_col_name	data_type	index_type
l_discount	         float	            indexed
l_quantity	         float	            indexed

Issues Resolved

#23

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[YANG-DB]

why is the 'All' needed ? the parameter nane and return value already indicate this is a pattern and may return a list of indices

[dai-chen]

Because there is another API FlintMetadata getIndexMetadata(String indexName) below with same name and argument list. I'm thinking get[All]IndexMetadata maybe better than getIndexMetadata[s]?

I was also thinking of providing single getIndexMetadata API for both purpose. But it seems confusing when we pass index name and expect single metadata returned.

[YANG-DB]

maybe getIndicesMetadata - anyhow the return signature also helps distinguish between them IMO ...

[dai-chen]

I can rename it to getIndexMetadata[s] if you have strong opinion? Or any better name? (getIndexMetadataWithPattern?)
IMO, it's intuitive to have different method name and also required by compiler... Thanks.

[dai-chen]

Will merge this for now and rename if we come up with better name. Thanks!