Reduce execution time of stream_unfetched_token_instances query

[vbaranov]

Motivation

The query https://github.com/poanetwork/blockscout/blob/da54650a4a559393d29a499a486b51d1477789ef/apps/explorer/lib/explorer/chain.ex#L2969 executes more than 1 hour on ETH Mainnet instance

This PR aims to increase the performance of this query.

Changelog

Decrease the subset before joining the tables

Before the changes query looked like this:

SELECT DISTINCT ON (t0."token_contract_address_hash", t0."token_id") t0."token_contract_address_hash", t0."token_id" FROM "token_transfers" AS t0
INNER JOIN "tokens" AS t1 ON t1."contract_address_hash" = t0."token_contract_address_hash"
LEFT OUTER JOIN "token_instances" AS t2 ON (t0."token_id" = t2."token_id")
AND (t0."token_contract_address_hash" = t2."token_contract_address_hash")
WHERE (((t1."type" = 'ERC-721')
AND t2."token_id" IS NULL) AND NOT (t0."token_id" IS NULL))
LIMIT 1;

EXPLAIN ANALYZE:

Planning Time: 0.520 ms
Execution Time: 0.851 ms

After the changes:

SELECT DISTINCT q."token_contract_address_hash", q."token_id" FROM (SELECT
  t0."token_contract_address_hash", t0."token_id" FROM "token_transfers" AS t0
INNER JOIN (SELECT * FROM "tokens" WHERE tokens."type" = 'ERC-721') AS t1 ON t1."contract_address_hash" = t0."token_contract_address_hash"
LEFT OUTER JOIN "token_instances" AS t2 ON (t0."token_id" = t2."token_id")
AND (t0."token_contract_address_hash" = t2."token_contract_address_hash")
AND t2."token_id" IS NULL AND NOT (t0."token_id" IS NULL)
) q;

EXPLAIN ANALYZE:

Planning Time: 0.423 ms
Execution Time: 0.592 ms

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 5d2a6440-9f2e-44ce-898b-8c1e50961ed9

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 75.107%

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Totals
Change from base Build 24fc869e-c854-4191-9ac9-0087b47c143a:	0.01%
Covered Lines:	5283
Relevant Lines:	7034

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💛 - Coveralls

[ayrat555]

@vbaranov tests are failing

[vbaranov]

@ayrat555 it is again test ethsupply returns total supply from DB. It is failed through all branches except master.

[ayrat555]

@vbaranov three tests related to token instances are also failing

[vbaranov]

@ayrat555 thanks, column name was wrong in the distinct select from the subquery. Fixed a7d9f0f