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In an off-list followup to bug #14745, Bob Jones complained that to_tsvector() on a 2MB jsonb value took an unreasonable amount of time and space --- enough to draw the wrath of the OOM killer on his machine. On my machine, his example proved to require upwards of 18 seconds and 4GB, which seemed pretty bogus considering that to_tsvector() on the same data treated as text took just a couple hundred msec and 10 or so MB. On investigation, the problem is that the implementation scans each string element of the json(b) and converts it to tsvector separately, then applies tsvector_concat() to join those separate tsvectors. The unreasonable memory usage came from leaking every single one of the transient tsvectors --- but even without that mistake, this is an O(N^2) or worse algorithm, because tsvector_concat() has to repeatedly process the words coming from earlier elements. We can fix it by accumulating all the lexeme data and applying make_tsvector() just once. As a side benefit, that also makes the desired adjustment of lexeme positions far cheaper, because we can just tweak the running "pos" counter between JSON elements. In passing, try to make the explanation of that tweak more intelligible. (I didn't think that a barely-readable comment far removed from the actual code was helpful.) And do some minor other code beautification. |
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