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I have a table in postgres which has rows like the following in a jsonb column called data:

{ "block": { "data": null, "timestamp": "1680617159" } }
{"block": {"hash": "0xf0cab6f80ff8db4233bd721df2d2a7f7b8be82a4a1d1df3fa9bbddfe2b609e28", "size": "0x21b", "miner": "0x0d70592f27ec3d8996b4317150b3ed8c0cd57e38", "nonce": "0x1a8261f25fc22fc3", "number": "0x1847", "uncles": [], "gasUsed": "0x0", "mixHash": "0x864231753d23fb737d685a94f0d1a7ccae00a005df88c0f1801f03ca84b317eb", "gasLimit": "0x1388", "extraData": "0x476574682f76312e302e302f6c696e75782f676f312e342e32", "logsBloom": "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000", "stateRoot": "0x2754a138df13677ca025d024c6b6ac901237e2bf419dda68d9f9519a69bfe00e", "timestamp": "0x55baa522", "difficulty": "0x3f5a9c5edf", "parentHash": "0xf50f292263f296897f15fa87cb85ae8191876e90e71ab49a087e9427f9203a5f", "sealFields": [], "sha3Uncles": "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347", "receiptsRoot": "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421", "transactions": [], "totalDifficulty": "0x239b3c909daa6", "transactionsRoot": "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421"}, "transaction_receipts": []}

I’d like to write a SQL query which selects all rows that have a null value for block.data but does not select rows which do not have a data field.

I’ve tried the following, and they all have failed:

SELECT * FROM table WHERE data->>'block'->>'data' IS NULL;
SELECT * FROM table WHERE data->'block'->'data' IS NULL;
SELECT * FROM table WHERE jsonb_extract_path_text(data, 'block', 'data') IS NULL;

It seems that in all of these cases, if the data field isn’t present, it passes the where clause.

3

Answers


  1. You can use the ‘@>’ operator to check if '{"data":null}' is contained in the data->’block’ path.

    SELECT *
    FROM tab
    WHERE (data->'block') @> '{"data":null}'
    

    Output:

    data
    {"block":{"data":null,"timestamp":"1680617159"}}

    Check the demo here.

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  2. Use two predicates to see if the key exists *and if the value is null

    where jsonb_path_exists(data,'$.block.data') and 
    jsonb_path_query_first(data,'$.block') ->> 'data' is null;
    

    Example

    with test as ( 
    select 1 id, '{ "block": { "data": null, "timestamp": "1680617159" } }'::jsonb  as data union all
    select 2 id, '{ "block": { "data": 1, "timestamp": "1680617159" } }'::jsonb as data union all
    select 3 id, '{ "block": {  "timestamp": "1680617159" } }'::jsonb as data)
    select t.id
    from test t 
    where jsonb_path_exists(data,'$.block.data') and 
    jsonb_path_query_first(data,'$.block') ->> 'data' is null;
    
    id|
    --+
     1|
    

    This query leads to an Seq Scanof the table as shown below

    explain analyze
    select t.id
    from test t 
    where jsonb_path_exists(data,'$.block.data') and 
    jsonb_path_query_first(data,'$.block') ->> 'data' is null;
    
    Seq Scan on test t  (cost=10000000000.00..10000003289.05 rows=167 width=4) (actual time=45.745..68.798 rows=1 loops=1)
      Filter: (jsonb_path_exists(data, '$."block"."data"'::jsonpath, '{}'::jsonb, false) AND ((jsonb_path_query_first(data, '$."block"'::jsonpath, '{}'::jsonb, false) ->> 'data'::text) IS NULL))
      Rows Removed by Filter: 100002
    Planning Time: 0.079 ms
    JIT:
      Functions: 4
      Options: Inlining true, Optimization true, Expressions true, Deforming true
      Timing: Generation 0.714 ms, Inlining 8.161 ms, Optimization 26.128 ms, Emission 11.304 ms, Total 46.308 ms
     Execution Time: 69.586 ms  <<<--------------
    

    If you have a very low number of rows with the data key defined, you may profit from the index

    CREATE INDEX idx_test2 ON test USING btree (((jsonb_path_exists(data,'$.block.data'))));
    

    The plan uses the index and is much more performant

    Bitmap Heap Scan on test t  (cost=551.35..2965.38 rows=167 width=4) (actual time=0.034..0.037 rows=1 loops=1)
      Filter: (jsonb_path_exists(data, '$."block"."data"'::jsonpath, '{}'::jsonb, false) AND ((jsonb_path_query_first(data, '$."block"'::jsonpath, '{}'::jsonb, false) ->> 'data'::text) IS NULL))
      Rows Removed by Filter: 1
      Heap Blocks: exact=1
      ->  Bitmap Index Scan on idx_test2  (cost=0.00..551.31 rows=50002 width=0) (actual time=0.025..0.025 rows=2 loops=1)
            Index Cond: (jsonb_path_exists(data, '$."block"."data"'::jsonpath, '{}'::jsonb, false) = true)
    Planning Time: 0.164 ms
    Execution Time: 0.052 ms  <<<--------------
    

    Used test data

    create table test as
    select 1 id, '{ "block": { "data": null, "timestamp": "1680617159" } }'::jsonb  as data, repeat('x',2000) pad union all
    select 2 id, '{ "block": { "data": 1, "timestamp": "1680617159" } }'::jsonb as data, repeat('x',2000) pad union all
    select 3 id, '{ "block": {  "timestamp": "1680617159" } }'::jsonb as data, repeat('x',2000) pad union all
    select id+3, '{ "block": {  "timestamp": "1680617159" } }'::jsonb as data, repeat('x',2000) pad from generate_series(1,100000) as t(id )
    ;
    
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  3. Just use 'null'::jsonb in the condition:

    select *
    from my_table
    where data->'block'->'data' = 'null'
    

    Other answers, however effective, are unnecessarily complex and hide the essence of things. The problem is that the 'null'::jsonb is not the same as Postgres null:

    select 'null'::jsonb is null;
    
     ?column?
    ----------
     f
    (1 row)
    

    You can use a simple btree index that can support the first query:

    create index on my_table ((data->'block'->'data'));
    

    Alternatively, the gin index can support the @> operator:

    create index on my_table using gin ((data->'block'->'data'));
    
    select *
    from my_table
    where data->'block'->'data' @> 'null';
    
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